Basic strategy tips:
Defensive:
Block up the holes - someone with a bit of skill can get units into your economy without being posed a single threat - you may be able to kill them relatively quickly but by then the damage will probably have already been done. Walking in the gap behind trees is particularly fun for them - put a tower up in the gap, or wall, or a building or something.
Try and keep towers in such a place that they will be able to do damage without gettting attacked back, I.E. behind the units defending your base - if you're playing macedonian cents, this can JUST give you the edge against carthage cents.
Try and keep some forces back to defend your base against a counter-attack, only send them out attacking the enemy when you're sure the threat of being attacked is passed.
A strong defence can be a good offence - if you're letting the enemy throw wave after wave at you and you kill them all with very few losses, then you're going to win in the end, since the enemy will have wasted his resources.
Offense:
Likewise, a strong offense can be a good defense, if the enemy doesn't actually have an army it's a win-win situation. Don't be tempted into letting your enemy pull the tip above on you. Remember how much a rush can muck up your game if it fails before trying it - if you're enemy has built equal forces AND towers then they have the home advantage of firepower and you'll likely come of worse - this can be a terrible waste of resources if they are able to splatter you and launch a counter attack - you'll also have got your villager priorites wrong for defending.
Try and find alternative routes into the enemy base - use them to launch diversionary attacks.
Attack from two sides - most people still haven't mastered fighting outwards on two fronts. Look what happened to the Germans in world war 2. Two fronts coming together on the other hand... (well, actually in WW2 there were 3, but that's irrelevant)
Try and keep your forces focused in one point in each attack (a 2-pronged attack counts as 2 attacks) This will increase your chance of breaking through, then swamping into the enemy base.
Try to take advantage of the stupid unit AI in AOE and try to draw enemy troops away during the battle, then turn around and surround them, this is particularly useful against elephants - it does require a lot of micro-management, but if you can master it, it will really give you the edge in a battle
Resources:
Keep a gold mine in your base until all the others have been exhausted - try and take at least your fair share of the others, and then when the enemy has run his out - you start mining and have 2000 gold still to spend.
If your defenses use a forest as a natural barrier, be careful about harvesting it - or your villiagers may unwittingly go and open up a great gaping hole - try harvesting inwards first - that will also give you more space for farms. You may not be using towers - but if you collect that stone then if the choson guy does overrun your base it won't be there for him - you can always give it to your allies who are.
Share resources - if you're using chariots give your gold to the hittite using Heavy horse archers - in a 3v3 water war, give food to the guy who's building up the landing force (if that's how you've chosen to play it).
Try to keep about 1000 food, wood and gold in stockpile at once - if you need to more base this could be invaluable.