Personally I've always found mortal chaos to be one of the easiest armies for Wood Elves to beat.
They have virtually no missile weapons (just throwing axes for marauder cavalry) and most chaos armies are made up of a few big blocks of quite expensive infantry.
However they have some of the best close combat troops in the game so it is vital to control the movement phase of the battle and make sure you pick your fights.
I'm going to assume that this is a pick up game with both players being unaware of the foe they will be facing and having balanced and typical armies of 1500-2000 points.
My prefered tactic against chaos is to use my scouts/waywatchers and glade/warhawk riders to march block and slow the enemy down. The Glade Guard are placed in the open, pouring fire into the chaos army. Unless they are a very magic heavy Tzeentch (even then our shooting will probably be more reliable and deadly) list the chaos player will be forced to advance as quickly as possible to engage. Otherwise they will be whittled down and picked off by the superior elven firepower.
The 1st targets to take out should be the fast and lightly armoured units such as war hounds, marauder cavalry, screamers and furies. Focus on causing panic checks on the hounds and cavalry, chaos only have average Ld so in the early turns don't be afraid to spread your firepower about, with luck these units will flee and get in the way and cause panic in their own force. The flying daemons need to be taken out ASAP, with the flying movement, and decent combat potential they can beat most of our units should they get to charge. They are well worth attacking with spells or the HODA.
Once you have taken out these fast throw away units you have to choose between shooting at either the enemy infantry or the heavier targets.
Chaos Knights and Chariots can be a major pain for us, chariots are less of a problem, they can't march and since we like hiding in terrain anyway then it isn't too hard to keep out of their way. To actually defeat chaos knights in combat is pretty tricky for us. The best method of taking them out is to bait them with glade or warhawk riders then hit them in the flank with a nasty combat unit like wild riders, treekin or wardancers. You can also stick a treeman in front of them and hope he passes his Ld test (far more likely with a BSB nearby) then hit them with some reinforcements, though this is not advisable should the unit be led by a character.
The other option is to draw them away from your less manouverable units (very easy on units with the mark of khorne) again bait them with skirmishers or fast units (ideally with a musician) and flee when they charge you, try to draw them into terrain or in such away that they will have to spend a turn reforming to get ack into the action. If the unit is really expensive you can even sacrifice a cheap unit to them. Scouts are good for this, walk right in front of them and they will be forced to charge, flee and you will get caught forcing them to move their full charge distance forward. If you are clever you can set up a nasty flank charge from wild riders.
The main bulk of the chaos force will be made up of marauders and chaos warriors, they are quite slow but very good at combat. However the trick is to only engage them after they have been weakened by our firepower and then wipe them out with combined charges. The best units to do this with are a fast cavalry (treekin work too) unit in the flank or rear and skirmishers in the front. Glade riders should only be used on marauders or 1 rank of chaos warriors, they die to easily otherwise, make sure the unit will be 3 strong so as to negate the enemy ranks and get the extra combat bonus.
You can use the threat of a flank charge to slow chaos units down, our skirmishers charge 10" so can stay in front of the enemy without being charged and the glade riders can threaten to flank the enemy whilst march blocking and shooting. Should the chaos unit turn to face the cavalry then they can move away and the chaos unit will have to waste movement turning back.
When you do charge a unit be aware of what happens in the next turn. Consider what will happen if the enemy hold, bearing in mind that most of our combat units can't choose to flee from a charge.
Spawn are a real annoyance, they have T5, are unbreakable and can't be march blocked. They should be targetted after warhounds and light cavalry or send a unit like wardancers or treekin/men to destroy them.
I'd make sure to always take plenty of anti magic protection against chaos, either that or target their mages. Unlike most armies chaos mages are still very nasty in combat so only take them on with suitably nasty units like wardancers.
Chaos can take daemons as special units but most of them are pretty worthless except flyers. We have plenty of magic attacks to negate their saves and in any case they tend to be quite slow, expensive with low toughness so won't be too much of a problem. Make sure you keep mages in forests or units if screamers are about as their slashing attack can be very deadly.
A final note on Daemon princes, they can do some serious damage with the ability to fly, terror and their combat potential. I'd cheerfully fire every missile weapon, including the HODA at these characters. They can't flee from combat so it's worth sacrificing a unit to take them down. I've "accidentally" left a 6 strong wild riders unit with the war banner in charge range of an exalted daemon before, the static combat res allowed me to survive the 1st turn then I was able to hit it with a unit of dryads. They also tend to be the army general so are well worth killing, it's vital that wood elves are able to outmanouver their foes and he can easily chase down fleeing troops and destroy the weak missile units.
That's all I can think of right now. If I remember something I've missed I'll edit this post.