My First attempt at a build so i apologize if it isn't as in-depth/flashy as most of the builds on here. Got Elise the day she came out and have been loving her ever since. Heres a link to a penta i got with her the other day: http://www.twitch.tv/zlainfury/b/342224401

Standard AP runes, but swapped mana regen for HP/lvl. Elise has no problem with mana so long as you don't go overboard casting in human form early. Latter in the game, mana is mostly a non-issue with her.

Again pretty standard masteries, swap the 3 in MR for armor if you're top.

Lich Bane is an amazing item on Elise. Cast Q to drop 10%+ HP and follow it up with an immediate attack for upwards of 300dmg, A+. Since her other half is a melee spider Zonyas lets you get in on a team fight and stay alive when they switch to the squishy spider. If the enemy team is heavy AD and not AP and isn't really building too much MR, ending with Nashars Tooth, instead of Abyssal is a decent option. Reardless of your form you have the ability to pump out nice dmg and the CDR helps for switching forms faster/Spider abilities which have long CDs due to being manaless.

Depending on distance to your target, send out your Spider first and while it's running pop off a neurotoxin, then your stun(Obviously if they are close start with Neuro then Spider). The reason you want the stun last is to give you and your spiders time to get to them when you change forms. Wait to pop your attack speed until you and your spiders are on the target or you'll be wasting some of the ability. Your spiders will always spawn in a fanned out formation behind you and take a second to get to their target. Once the AS wears off, or you think they're low enough, Bite em for the kill. If you need to get away Rappel and try to land on minions opposite where they are. If they stay and run to between mid and your turret land on their minions and run up and around river. Another thing you could do in a close fight is drop Ignite on them, then just Rappel and hang out avoiding damage. Utilize your spiderlings when low. Turn into a spider and just walk around the minions, your spiderlings will auto attack them giving you a small amount of life back from their attacks.

As far as some things vs individual champs:

Morgana, If you cant get to a side to avoid a q, and you're in human form, switch to spider and simply run straight away, Her q will hit a spiderling, not you. Also try baiting out Qs, side step and retaliate with a Q/Spiderling combo of your own. Rappel will also stop her ult from popping on you.

Zilean, O he put a bomb on you? Wait till it's about to explode and rappel.

Karthus, O he's ulting? Wait till it's about to go off and rappel.

The rappel thing is pretty much the same for Lux/Draven/Ez/Ashe ults. Infact vs an Ashe ult, if you spider and let your spiderlings fight minions with you off to the side, the Arrow could pop on them leaving you fine, if you can position it right.

Also note you can make your Human W(The spider Bomb) Leap to your target so long as it is active and you use your Spider Q on your target.

And last but not least, Ward, Ward, Ward. Between Flash Rappel and Stun Elise can get away from a lot, but obviously advanced knowledge makes it that much easier. Be sure to utilize wraiths on either side, be it you pushing in or them. If they are up Rappeling to them is a great way to run without using Flash.