General Army Notes:
High Elven missile fire is all at +1 to hit, and High Elven Mages may reroll any failed spell checks. This means that High Elves have very effective missile fire, and very potent wizards.
Foot Units:
Spearelves: You must take at least two mandatory units. Spearelves have a better than average set of stats, giving them a decent chance in an assault.
Archers: You must take at least one mandatory unit. Archers do a good job of giving supporting fire to other units, especially with their +1 to hit. Elf missile troops are better than those of other races, so take advantage of this superiority. Use them to support your other troops and to weaken and/or eliminate threats from your opponents.
Cavalry/Chariot/Flyer Units:
Silver Helms: Silver Helms give you an excellent fast assault unit. Remember to back them up with Reavers (for supporting fire and extra weight in an assault).
Ellyrian Reavers: Ellyrian Reavers are a good support unit, able to fire their bows and charge into combat both. Reavers are a good unit in defense as well, because they can support other units, getting off many shots with their bows, and then be used to counter charge assaulting enemies in your next turn. Reavers' low save makes them somewhat vulnerable, however, so keep this in mind.
Chariots: Chariots are best used adding their weight to an assault, or on the flanks of your line.
Giant Eagles: Giant Eagles can fly up to 100cm each turn, making them a good harassment unit and good for lending their weight to an assault. They are, however, very vulnerable so be sure to consider this when making an assault.
Dragon Rider: The Dragon Rider's high stats and ability to cause Terror can be very devastating. The Dragon Rider can do a lot of damage, but if the Rider suffers 3-5 unsaved wounds at the end of a shooting or HTH phase, he is considered seriously injured and his stats drop to 3 Attacks and 3 Wounds. The Dragon Rider is only really useful in larger battles. You may want to think about whether the points could be better used for other troops before purchasing this very expensive unit. In most battles you should just skip the Dragon Rider and buy other units (for instance, you can buy three units of Silver Helms and have 20 points spare).
War Machines:
Repeater Bolt Thrower: The Repeater Bolt Thrower is very effective for supporting other units, with its 3 shots at 3+ to hit. It is very vulnerable to assaults, however.
Characters:
General: The General's ability to give orders is very important, so he should be kept in a fast-moving unit, where his +2A and command is best used. You should think about where along your battle line he can do the most good.
Hero: The Hero is very effective when lending his +1A in an assault and giving orders. You can only have one per 1000 points, so he needs to be in a unit where he can use his abilities to the most effect.
Mage: The High Elf Mage has some of the best spells in the game. The Mage gives no attack bonus, so make sure he is with a unit that can protect him.
Giant Eagle Mount: The Giant Eagle makes an excellent mount for a character, as it allows the character to move from unit to unit and lend its attack bonus (as well as the Eagle's Attacks) to the unit in an assault.
Dragon Mount: The Dragon Mount can prove to be very useful in causing trouble for the enemy and lending its weight to a unit in an assault, but its high cost means it should only be used (if at all) in large battles. The 100 points can be used to much greater effect for more units.
Chariot Mount: The Chariot is a cheap mount that provides your character with some extra hitting power and mobility.
Special thanks to Dalkwave for his help on this tactical overview!