When you start a game that you did not play much before, like a particular scenario map, you don't know or remember where are the enemy bases. A simple way to know is to check for colored flags (use your base as example), observe the the map and check flags presence to know who is where.
Create units faster
Each unit has it own shortcut letter on the keyboard, in a way to train units faster than clicking . Building also have their letter, units can have the same letter but you must have the appropriated building selected . Rather than press the key 15 or 20 times, you can bind this letter with SHIFT key; this will create units 5 by 5, so to fill in an archery range, or a barrack, hold shift and press the shortcut letter 3 times only, to fulfill the unit's queue.
Your opponent might notice that you have a civilization that specializes in particular units (Vikings: Infantry, Franks: Cavalry...). He might then expect that you will base your army with theses units, and only use the counter unit to theses units. You need then to create the complete opposite to the units that your opponent expects you to use.
For instance take franks, a good cavalry civ, and make archers. Your opponent think you will make horses due franks civ, so he will make pikemen, and your archers will kill them easily.
Using this strategy can be risky, because your opponent may anticipate your strategy and build the counter to the radical unit that you are planning of creating.
Better ranged units
When both you and your enemy use archers units, its a range battle. If you have longer range you can pull your archers back just when the enemy can hit you. This will make his men follow your units, letting them out of range . Stop your archers at this moment and kill the followers. Doing that repeatedly is called Hit and Run, but it is an unfair way to take advantage on enemy. It can be forbidden for some types of games.
Predicting enemy retreat
Most of time, when your enemy is loosing, he will pull back his units to save them. if you can guess that moment, or even react fast just after he pulled them, move your units closer to them, in a way that you can hit them while they are running away.
It is possible to garrison units inside barracks, archery ranges, stables, siege workshops, castles ( trebuchets ) and docks . But you can only garrison units that are created from the building, what you have to do is set the gathering point (I) on the building itself.
This will make the newly created units automatically garrison inside the building when they are ready (most of time, 10 spots by building). Note that your building must have at least one free terrain unit close to it for the unit to spawn, even if it is garrisoned just after.
Once un-garrisoned, some units cannot be garrisoned again; siege units, most of time... This is good if you want to build up an army before letting them out, and keep it hidden in buildings, letting the enemy without any clue on how many are your units, and how powerful they are. If the building is destroyed, all the units are un-garrisoned.
Distract your enemy with buildings: If your enemy is about to attack you on land, quickly build several cheap buildings, such as houses, all over the area of land between you and your enemy.
When your enemy reaches your distraction buildings, most likely he will attack them, moving from building to building destroying them. Not only this will keep him busy and make him waste time, but it will signal his position to you, but it can also tell how far the enemy is, and what units types has his army.
The larger screen size you use ; the more you can see (maximum 1280x1024 on aok/aoc, and up to 1920x1080 for Forgotten Empires.
This makes is easier to see what your enemy does, as well as making it easier to build. When you first change it can be harder, but you'll get used to it. Obviously saboteur shooting is harder on bigger sizes but it compensates with more sight, actually noticing they are sabbing.
Who is watching
Sometimes when players resign, you can see that they have the turtle sign, that means they are still in the game and lagging it. But other times you might want to know who of the dead/resigned players are watching. You can do this by checking the chat, here you can change who to write to. If you can click (the box) to talk to a resigned player, that means he is watching.
More tricks ideas ? Feel free to comment suggestions below.