Dark Eldar forces are like paper. Brittle paper. Not only are they brittle, their point cost is expensive. I mean really how long can you expect your Dark Eldar to survive head on heavy fire and still win the game without getting decimated.
The answer is, you cant. Dark Eldar work best when they can strike hard and fast. It is after all why we have so much speed on our side. The longer we stay on the table vs a good opponent with a good list, the worse off we are.
One solid answer is designing your list around reserves. While reserves are not my most favorite thing in the world, Dark Eldar can do it with style. Aerial Assault rules, Aether Sails, turbo boosting Reavers, Webway Portals, and Duke Sliscus make lists that can capitalize on a reserve list.
Why a reserve based list? Dark Eldar hit hard, and die to a protracted engagement. Lets shorten the game, hit the enemy where we choose in a surgical strike and criple the enemy, before we loose too much of our force. Dark Eldar are arguably one of the strongest hitting forces in the game. When they hit they wipe out sections of the board consistently. Reserves take advantage of this nicely.
Deploying: I played so many of my early games deploying all my forces and trying to survive long enough to deal damage to my opponent. Night Shields and Flickerfields, while helpful, are not the answer. Not that I wont use them, but their use alone will not save your vehicles. I have found that even trying to set up out of range of the enemy can very quickly turn against you, and most of your precious army get shot to pieces.
Pain Tokens: Pain Tokens are nice, and help you survive in late game, however too many people are making their lists counter productive to getting pain tokens, but that is another topic. Pain Tokens will carry you through, as long as your units capable of getting them, are the ones killing the non-vehicles units. Do not rely on getting pain tokens to keep your units alive. They are a secondary tactical design element.
Control: Dark Eldar have many options when it comes to reserves, and can literally strike from so many locations anywhere on the board. This gives you a very nice sense of control on the tabletop. Not only do you get to take control of the table top, your forces get to engage first, get shot later.
So Many Options. As a Dark Eldar player you have so many options at your disposal for reserves. Even then, with a webway or two, your flexiblity on the table top increases exponentially. This much speed, and not knowing where the Dark Eldar will strike, often puts players into a defensive mode right from the start. This is ideal for many play styles, especially mine.
The point is, Dark Eldar really take advantage of reserves. They suffer from prolonged games because of how fragile and expensive they are. Strike hard, strike fast, and leave the enemy crippled wondering what the hell happened in the aftermath. Giving the enemy only 3 rounds in a 5 round game to deal with you is advantage that Dark Eldar can capitalize on better than almost any other army out there.