Gym leader Tactics
Here are your best bets for beating the Pokemon Gym Leaders: 

Brock: Use water or grass Pokemon against this rock trainer. 

Misty: Use grass pokemon or electric to defeat this gym leader. 

Leiutenant Surge: use a rock or ground pokemon and he's toast. 

Erika: Use fire pokemon or flight against this plant lover and shatter her dreams of fighting you again. 

Sabrina: Use bug or ghost against this psychomaniac and she is a goner. 

Koga: This poison pokemon lover will be deafeted easily by using psy pokemon. 

Blaine: This fire trainer will use fire pokemon against you so you can use water or ice. 

Final boss: Giovanni, the team rocket leader, is the final gym leader - use water, ice, plant and he's out of there (and talk to him again and he says something to you). 

In the Indigo Plateu you find 5 pokemon masters. Use this pokemon lineup for a better chance of success: 

Plant 
Water, Plant, Flight, Ice 
Psychic 
Ice or Dragon 
Your choice 

Celadon City Slot Tip
All of the slot machines have different odds, which change every time. Play each machine 4 times, and if it hits 2 or more, then stick with that one, because it's probably hot. If the slow machine forces you really close to a 777 or Bar Bar Bar it will probably give you one or the other soon. 

Cut Down Grassy Areas
Once you get the HM "cut", your pokemon can cut down grassy areas. Use "cut" and it will cut a two by two area. Wild Pokemon will not attack you in the area you cut. 

Duplicate Items
To get 99 of any item, first put the item you want to multiply in the sixth item slot. Now use FLY and go to Viridian City. Talk to the old man almost all the way north of the town. When he asks if you are in a hurry, say no and watch him catch a weedle. Right after that, fly to Fuchsia city. Make your way south and, when you you reach the water's edge, use SURF and make your way to Seafoam island. When you get to seafoam island, swim up and down the beach until you run into a Pokemon called "M" (Missingno.) Once you see him, kill him or run away. DO NOT  CATCH HIM! After that, you should have multiple copies of your sixth item! 

Easy Level Gain
To easily gain experience, simply switch the Pokémon you want to train with the top Pokémon on your list. When you go into battle the Pokémon will pop out. You can then switch to another Pokémon. Once you win the battle the beginning Pokémon and the fighting Pokémon will both gain experience points. 

Extra Damage
While fighting, select your desired attack. When it says you are going to use that attack (eg. "Venusaur used Razor Leaf"), hold down the A button until your opponent's life stops draining. If your timing was right you'll do more damage than usual. 

Fight Land Creatures in Water
To fight creatures that normally live on land in water, go to the water around 
Cinnabar Island or Seafoam Islands (The only place you can be randomly attacked while swimming.) Swim along the shore of any island and you will fight land creatures. (You have to swim on a place where the square is half land and half water.) Instead of fighting a set group of monsters in the water, you actually fight monsters from the last place you were. So to get safari zone monsters, you need to go to the section of the safari zone with that monster, let your time run out, then go to the coast. Using this trick I have caught Kangaskhan, Scyther, and Tauros! 

Fight the Elite Four More Than Once
First collect all eaight badges. Then go through victory road. Next beat the Elite Four. After that your Pokemon will become Hall of Famors. Then the credits will show, following by your game resetting. Go back to your Continued Game. You will be back at home. Then fly back and beat them again and again. This helps your higher leveled Pokemon because you can still get a few thousand exp. 

Get Mewtwo
After you have gotten all eight of your badges you need to go beat the pokemon league. The credits will roll and then the menu pops up again. DO NOT RESET THE GAME!!!! Go to Cerulean Citygo north to the brige where you faced 5 trainers before the path to Bill's house. Turn left and there will be a large area of water. Surf until  you get to a cave. You will find Mewtwo in that cave. 

Get the Mimic TM
Buy a Pokedoll in Celadon Mart, then FLY to Saffron and talk to the Copycat. She will give you the Mimic TM. 

Invisible Walls in Koga's Gym
If you look at the floors in Koga's Gym you will notice it has lines. In some spots you  will see little gaps in the lines. These "gapped lines" are the invisible walls. 

Level 100+ Pokemon
Follow these instructions to catch pokemon with really high levels. Talk to the old man in Viridian City who shows you how to catch pokemon. 

Fly directly to Cinnabar Island. Put your strongest pokemon on your team. You might want to save now. Surf on the shore where the land touches water.  Keep Surfing until you run into one of these strong pokemon. These pokemon are well above level 100. Unfortunately, they will revert back to Level 100 if they gain any experience points. Rare Candy is the only thing that raises their level. They can be used in a two-player battle without losing their level, so have fun liquidating your friends! NOTE: This is a bug in the game. It will not work in newer versions of Pokemon and may crash your game if you try it. 

Secret Coins
At the Game Corner there are secret coins lying around. You can get these coins by walking around and pressing the A button. 

Non-GameShark Tips

Fish on Misty's Gym:
You need: Old Rod 
1. In Misty's Gym, face the statue and use Old Rod. 
2. You should be able to fish. 

Stand on top of a tree:
You need: Cut 
1. Cut down the tree that you can cut. 
2. Stand on the spot where the tree was and go to menu and save. 
3. Turn the game off, then turn it on again and continue. 
4. When you start the game, you should be on top of a tree. 

Easier way of catching Pokémon:
You need: PokéBalls 
1. When you are in battle with a wild Pokémon, Throw a Pokéball. 
2. Hit the A buttion and press the right arrow key repeatedly. 

Evolving Pokémon with Stones: 
Here's the Pokémon to use the stones on: 

Pikachu + Thunder Stone = Raichu 
Eevee + Water Stone = Vapereon 
Eevee + Thunder Stone = Jolteon 
Eevee + Fire Stone = Flareon 
Clefairy + Moon Stone = Clefable 
Nidorina + Moon Stone = Nidoqueen 
Nidorino + Moon Stone = Nidoking 
Gloom + Leaf Stone = Vileplume 
Growlithe + Fire Stone = Arcanine 
Jigglypuff + Moon Stone = Wigglytuff 
Vulpix + Fire Stone = Ninetails 
Weepinbell + Leaf Stone = Victreebell 
Poliwhirl + Water Stone = Poliwrath 
Exeggcute + Leaf Stone = Exeggutor 
Shellder + Water Stone = Cloyster 
Staryu + Water Stone = Starmie 

Trading Evolution:
Some Pokémon won't evolve until you trade them. Here's the list: 

Machoke evolves into Machamp 
Kadabra evolves into Alakazam 
Graveler evolves into Golem 
Hauter evolves into Genga