"You are doomed, DOOMED I tell you! Tonight we shall feast on your soul." -Lord Zuol

The Rush:

This is a tactic a guy at the store told me he saw used against Orks, and tells me it made very short works of them.
What you need is at least 4 raider transports, 6 reavers, 2 squads of warriors (raider squads of 10 warriors each), a squad made of a dark eldar archon, 4 incubi and 5 warriors and finally another squad of 10 wyches). All these squads are on board raiders, and there are 6 Raptor motojets in front of the raider for cover.
All you do is move forward. You can chose to use the Reaver's turbo boost to get even closer to the enemy units. Your opponent probably didn't have his units ready for a 2nd turn assault, and has 1 turn to be prepared. On the second turn, your remaining motojets attack, your troops are dropped off and the raiders start to fire with their dark lances, or the more mobile blasters and splinter canons.. If you can afford it pointwise, get more incubi to support the DE archon, as these guys are worth their cost in battle, thanks to their good armor(by DE standards), and power weapons. It's a risky move, but with the DE, you just can't sit around and wait to be shot at...

Flanking:

Using a flanking technique with the DE can sometimes be somewhat of a problem against armies which depend on armor and tanks, as the DE army is very lightly armored and armed. Nevertheless, thanks to the reavers and the Grotesques, flanking can be a good idea. Simply put a wall of Grotesques in front of your main force, and have the jet bikes and a raider with Wyches ready to hit the flank. The best way for your opponent to take out the Grotesque wall is to go hand to hand, or long range with tanks. If he's going to se tanks, equip some of your Raptors and Wyches with anti-tank weapons, and watch your opponent retreat or be rapidly destroyed (God bless antigrav technology and turbo boosters).
If the opponent sends hand-to-hand squads, have your incubi(these guys ROCK) and your flankers ready, and when he finally hits your Grotesque wall, he's trapped and quickly destroyed. After that, have the Grotesque wall advance, with your troops behind, ready to pounce. Another cheap way to flank your opponent is to deploy all of your units in raiders on ONE side of the table. Your opponent, to counter this, will probably do the same, putting the vast majority of his units on the same side as yours. On your first turn, move all your units accross the table(sideways,not forward!), and watch your opponent scramble mady to try to counter this move, as his whole flank is now open.

How to use Warp Beasts and Talos':

This particular tactic is designed for armies over 1000pts, as you have to be willing to risk quite a few of your troops to get it to work. First off, you need the usual DE lord and his incubi/warrior escort to be in a raider. Have 10 wyches aboard another raider, and two more raiders filled with 9 warriors and a Sybarite. Give one of the sybarites a Warp Portal(50pts), and do all that you can to keep him alive. The Warp beasts and the beastmaster are in reserve. Have a large enough force of jetbikes in front of the raiders as a wall, and to disrupt the enemy defenses before the arrival of the beasts. Rush with All the jetbikes and All the raiders. As soon as you reach the enemy lines(Hopefully the jetbikes took the brunt of the attack), drop off the troops and cause as much confusion as you can. The incubi and the wyches are especially good at this. Having equipped the wyches with quality weapons like blasters helps a lot. The warriors should all have one goal: protect the sybarite with the Warp Portal. As soon as he puts it into play, try to call upon your reserves. The warp beasts and other Wych squads arrive and help finish off the enemy. Another good idea is to have a Hoemonculus waiting back with a small escort in case things screw up and your troops begin to flee. Adding a Talos or two to the reserves and bringing them up like that makes them quite useful.

Basic tactics:

Here are some basic thing that a player shouldn't forget if he is to win a game with a DE army:

1) Your army is the most mobile in the game, take advantage of this! Keep moving, and keep your units free of weapons that can't let a character run AND shoot. Except for your anti tank squads which should be under cover.

2) Use the scenery for cover! Most DE units have an armor safe of 5+, while under cover they can have a better safe (up to 4+ i think), as you will take a LOT less casualties this way.

3) Put your HQ, Elite units(mandarkes excepted) and warrior squads aboard raiders. Unless the table gives a lot of cover, you will need all the mobility you can get, and raiders are quite good at moving your units at great speed.

4) Try to take out the enemy's anti-tank and heavy tanks first, as if they go down and you still have your raiders, you will become the biggest pain in the ass your opponent has ever seen ! Ravagers do this quite well, but have weak armor...

5) Incubi rule, except for their cost in points. Equipping your troops with as many splinter canons as you can is also a great idea.

6) The talos is slow, and unless you're planning on camping(which is pretty stupid with the DE) or bringing him in from the reserves with a warp portal, he's not worth the 100pts. Giving him Grotesque cover works too.

7) Did i mention that a squad of Grotesques led by a hoemonculus works wonders as a living wall?

8) Keep your weapons mobile: blasters, splinter cannons and slpinter rifles allow your units to run AND shoot. This is a MUST for the dark Eldars armies.

Battling other armies:

Space Marines, Sisters of Battle, Space Wolves, Chaos, Blood Angels, Ultramarines etc...
Most of the units have quite good armor saves, and usually have a lot of heavy weaponry/tanks.
Against a heavy army like that, use splinter cannons, blasters and other weapons that will allow you to take out heavy armor. Go easy on the use of dark Lances as they are quite good, but you cannot move if you use them.

Orks: These guys can't shoot very well, but are quite potent at HtH combat. Be ready to handle their fast attack units with blasters and Dark Lances, and then mop up the rest of their army with long range weapons and hit-and-run charges with Reavers and Raiders/Ravagers.

Necrons: These guys are tough. I haven't gotten to play them yet, but have seen them in battle numerous times. Perhaps being a bit more cautious and having a lot of armor piercing weapons can help.

Tyranids: These guys have one major tactic: the wall rush. They put their weaker units in front of the stronger ones and rush. Against that, using Ravagers with desintegrators, blasters, lots of splinter cannons and other template weapons will help. Wyches and Scourges are rather useless, as you do not want to engage the tyranids in HtH combat, and they will not run away. Beware the venom cannons, and keep your big units into cover. Plasma greandes will help if the 'Nids come too close to your army and are still under cover.

Eldars: These guys can more than handle their own, so be careful. They have the most balanced army in the game, so beware a devastating quick attack and protect your raiders from all the Eldars' long range anti-armor weapons.