Semi-Advanced Tactics
Advanced Tactics
Character Specific Special Tactics
Getting Back on the Stage Tactics
Edge Guarding Tactics
Using Throws in Combos

Semi-Advanced Tactics:

Rolling - Hold L or R and press right or left. You will roll in that direction and avoid attacks. This is an invaluable defensive technique that can be used at many different times during play.
Air Dodging - Press L or R in air to avoid attacks. You can also press a direction to dodge that way. This is useful for avoiding attacks and sometimes getting back on the stage, however it has a major disadvantage in that it leaves you very vulnerable after the dodge.
Ground Dodging - Hold L or R and press down. You will avoid any attacks for a small period of time and not change your position. It is much better for characters who can recover from it faster like Fox/Falco and Shiek. It is also useful for slower characters who are fighting faster characters. However, using it at the right time can be difficult, especially when fighting an intelligent player.
Catching items in air - While in air, press Z when you come in contact with an item to grab it.
Falling faster - If you press down while in air, you will fall faster than normal.
Wall Jumping - Many characters can wall jump. Basically any character that isn't massive or doesn't fall slow can wall jump. Run towards a wall, jump at the wall, and at the moment you touch the wall, hit the joystick in the opposite direction to bounce off the wall upwards. In Fourside you can walljump all the way up between the buildings.
Short Hop - Any character can short hop by quickly pressing jump. If you press jump quickly, you will jump a much lower height than normal.
Power Shield - Every character has a normal shield and a power shield. To use the power shield, press L or R lightly. You will be surrounded by a larger shield than normal, which is called the power shield. If you activate this at the moment a projectile hits you, the projectile will be reflected back. That may be the most difficult technique in this game though. It is so hard to do that I would imagine it is almost impossible for anyone to get it 10 out of 10. I rarely ever see it done once.
Dash Dancing - Dash dancing is when you run in one direction, then push the joystick hard in the other direction to run that way, then continuously push the joystick in the opposite direction each time. You will run back and forth in place. This technique is more useful when you are about to be attacked and you run back, then run towards them again, instead of just using it in place.

Advanced Tactics:

Wave Dashing - Jump and immediately air dodge diagnally towards the ground. It takes a lot of practice, but you will make a quick sliding motion across the ground. This is like rolling but faster and can become extremely useful.
L Cancelling - Hit L or R at the moment you land from using an aerial attack. The recovery time will be shortened.

Character Specific Special Tactics:

Luigi - Luigi's forward+B move has a random chance to be much stronger and go much farther. Luigi's taunt can do damage to your opponent.
Bowser - Bowser's Claw (foward+B) can grab the opponent if used at a close range, and you can savagely bite them several times depending on their percentage.
Peach - Peach can float in place in air for a few seconds. While in air just hold up and she will float. Peach also has a very rare chance to pull out a Beamsword, Mr. Saturn, or Bomb-omb, when using down+B.
Yoshi's Eggs - Yoshi can control the height and distance of the thrown eggs.
Captain Falcon/Ganondorf - Their up+B move can grab opponents with their shields on. Ganondorf also has the best taunt.
Ness's PK Thunder - Ness can control his PK Thunder, and can cause damage by hitting himself into other people with the PK Thunder.
Kirby's Hammer - Kirby's forward+B is different on the ground and in the air.
Samus's Missiles - By pressing forward+B or pressing forward+B hard you can use normal missiles or power missiles.
Zelda - She can control the distance and direction of her forward+B and up+B moves.
Shiek's Needles - Shiek can hold her needles and walk around with them by charging them and pressing L or R.
Shiek's Existance - Shiek can not make sense by existing.
Link/Young Link - They can control the power and angle of their boomerangs. They can use Z in air to attack with the Hookshot. They can also use a two-hit forward smash by pressing A right after the first forward smash is done.
Pichu's Electicity - Pichu hurts itself from using electrical attacks.
Pichu/Pikachu's 4th Jump - They can use their 3rd jump twice if you quickly press the joystick in a new direction after using the up+B.
Jigglypuff - Jigglypuff will instantly die if her shield is broken. If you hit up right after using Pound (forward+B), Jigglypuff will use Pound upward slightly, called Rising Pound.
Mewtwo - Mewtwo can go far really fast if you jump and shoot the Shadow Ball. If you use Confuse (down+B) on an opponent that is already confused, they will go flying. You can't Flip (forward+B) an opponent who has their back to him. If you Flip (forward+B) a projectile, it won't hurt your opponent, but pass right through them. Also, he almost doesn't look like he moves when you use his ground dodge.
Mr. Game and Watch - His Bacon (B) is extra powerful at a close range. His Judgement (forward+B) can do very different things based on the number. If you catch 3 of Ness's PK Flash in the Oil Bucket (down+B) and throw it at an opponent who has their shield on, Mr. Game and Watch will fly off the screen and die. Anyone he hits while doing this instantly dies.
Marth/Roy - They can do several different combos using forward+B. Use forward+B the first two hits, then use other directions for the last two hits. Marth is more powerful with the tip of his sword, and Roy is more powerful with the inside of his sword.

Getting Back on the Stage Tactics:

Dr. Mario/Mario - They can use the Cape (forward+B) to move a little closer in air.
Luigi - He can use the Rocket (forward+B) to get near the edge, then use the up+B.
Peach - She can float by holding up.
Captain Falcon/Ganondorf - This usually won't help Captain Falcon, but it is very useful for Ganondorf. If you jump twice, then use the Falcon/Warlock Kick (down+B), you will be able to jump again and use the up+B. This technique will almost always save Ganondorf. But Captain Falcon's kick goes too far, so it is useless most of the time unless he is really high.
Falco/Fox - They can use the forward+B move to get back on. This won't help them get back from a farther distance, but it can help you avoid being edge guarded.
Samus - Just like in the Metroid games, she can use Bombs (down+B) to bounce herself upward. If you can master the timing of the bombs in this game, you can get back on from almost anywhere with Samus. She can also grab the side of the stage by pressing Z in air.
Zelda - She can control the distance of her up+B move, and becomes temporarily invisible. Both help prevent edge guarding.
Shiek - Transform into Zelda (down+B) and use her up+B move.
Link/Young Link - They can use the Hookshot by pressing Z in air to grab the side of the stage. Also, if you have a bomb in your hands, you use the up+B while in air, and the bomb goes off while you are still in air, you can use the up+B again. This is rarely used because it is so hard to time, but it is possible to get back on doing this.
Pichu/Pikachu - If you use the up+B and immediately press the joystick in a new direction, they will use the up+B again.
Jigglypuff - Although it is almost never necessary, she can use Pound (forward+B) to move insane distances horizontally in air.
Marth/Roy - Use forward+B to move a little closer in air. Just make sure to leave enough time between each strike or you will start a combo, which will probably get you killed.

Edge Guarding Tactics:

Smashing Down - Many characters have an attack that will send an aerial opponent directly down until they hit something. Doing this while they are off the stage will instantly kill them. It is easiest to do this with Falco, Captain Falcon/Ganondorf, and sometimes Marth.
Grabbing the Edge - Any character can grab the edge. If you are holding onto the edge, then your opponent can't when they try to get back on.
Dr. Mario/Mario - They can use the Cape (forward+B) to turn an opponent around who is coming back on. If the opponent has used their up+B move and becomes turned around, they won't be able to grab the edge. This works best against Fox/Falco's and Captain Falcon/Ganondorf's up+B moves.
Bowser - Bowser could probably like cheap you up with his Fire Breath (B) or something.
Peach - If you can throw Vegetables (down+B to pick up) accurately, you can really cheaply kill people who are trying to come back on the stage.
Yoshi - Aerial forward+A will send them straight down.
D.K. - Aerial down+A will send them straight down.
Captain Falcon/Ganondorf - Aerial down+A will send them straight down.
Falco - Aerial down+A will send them straight down. He is the easiest character to do this with.
Fox - I am saying this because I have seen Adam do it. With Fox you can jump off and use an aerial attack, probably either the aerial backward+A or aerial up+A, to kill your opponent and still make it back on with the up+B (assuming you don't miss).
Ness - Send a PK Flash off the edge where your opponent is coming back on. It can be almost impossible to avoid with some characters. Ness can also use aerial down+A to send them straight down.
Samus - Use all projectiles. Samus can also use aerial down+A to send them straight down.
Zelda - Jump off and use the aerial back+A on your opponent, then use up+B to get back on.
Shiek - Needles used in the right way can kill characters that are bad at jumping back on.
Link/Young Link - Use projectiles. Link can also really cheaply kill people using the up+B because of its range when you are standing right on the edge.
Pikachu - Jump off and use the aerial up+A on your opponent, then use up+B to get back on. I don't know if Pichu does the same thing.
Jigglypuff - Jump off and use any aerial attack. Jigglypuff will probably be able to get back on no matter how far off you jump.
Mewtwo - Shoot the Shadow Ball. You can also put your back to the edge and charge up the Shadow Ball, which could get people caught in it trying to get back on. He can also use the aerial down+A to send them straight down.
Marth - The Poke (weak down+A) can cheap people up if you can hit them with it. Marth can also use the aerial down+A to send them straight down. I'm not sure if Roy can do the same thing.

Using Throws in Combos:

The following combos may only work if the opponent is around a certain percentage.

Dr. Mario - Down throw and up smash
D.K. - The infamous grab the opponent and suicide.
Captain Falcon/Ganondorf - Up throw and up+B. I don't know how well this works with Captain Falcon, but I always use it with Ganondorf.
Falco/Fox - Down throw and Reflector (down+B) or down smash.
Ice Climbers - Grab the opponent and use the C-stick to smash with Nana.
Kirby - If you are standing on the edge, you can grab the opponent and use the down throw to suicide.
Shiek - Down throw and weak up+A kick.
Jigglypuff - Down throw on a breakable object and Rest (down+B). Breakable objects are things like the blocks in Mushroom Kingdom and the breakable ice in Ice Mountain.
Mr. Game and Watch - Up throw and up+B.
Marth/Roy - Backward throw and forward smash (forward smash backward... in the direction they are).