The big thing, that even caught out one of our top players at the GT the other weekend, is the existence of a cheap Enchanted Item- the Amulet of Light, which can be taken by the champion of any elite unit, that grants magical attacks to the unit. So if you have Wraiths, beware! Also note that most fighting characters, and their Dragon Prince cavalry will all be immune to fire attacks- including any monstrous mounts, so don't take Balefire Spike or Hellfire Banner unless you think you can avoid them fighting the enemies immune to fire. As all High Elves have Always Strike First, the Nightshroud is brilliant on any combat character.
As for the enemy army, you can expect a few different types of build. In no particular oder:
1) Magic-heavy. This could mean Teclis if your group allows special characters, who gets IF with casting rolls that meet the value of the spell and contain doubles (i.e. 2-2-5-6 is IF for Wall of Fire which needs 12+, but 2-2-3-3 would not be). Or else it could just be an Archmage, probably with a bound magic missile, with support mages, possibly including one on a Dragon. Just let him blast your inconsequential blocks of troops and protect your heavy-hitters and you'll be fine. He'll probably put a lot of effort into Drain Magic, which should only worry you if you absolutely RELY on invocation/ Danse-spamming.
2) Star Dragon lists. Usually either with a BSB containing Battle Banner (+D6 to combat res!) and a caddy, or 2 level 2 Mages, so not impressive magic. Try and bog down the Dragon with two or more units containing champions, and he'll be stuck forever most likely.
3) More balanced lists- will contain a mix of infantry and cavalry, and probably a Prince. If they have a Prince on foot, be very careful, and he will probably have Killing Blow and an item for re-rolls, so will be able to kill your Lord very easily with his ASF.
Remember that with their ASF fighting them can be difficult. A Black Coach with its impacts and high toughness/ ward can do very well against High Elves, as can Wraiths as long as you avoid their magical attacks. Hope that's help for a start!