To perform a successful ambush a good strategy is to sacrifice units on the opposite flank to fool the enemy into looking the wrong way and then attack from behind.
Engineers
It is always worth keeping several Engineers out in the field with your main army as they come in useful when you come across something that could be captured.
Focus on the Objective
In Single-Player missions it is not necessary to control the whole map to win, make your task less daunting by just focussing on the objective.
Jeeps
Before deploying jeeps make sure you have fully scouted their route because although they are great for transporting infantry they are easy targets for enemy attack.
Scouting
To be successful in both Single-Player and Multiplayer you must scout your enemy. Knowing what the enemy has built gives you the chance to buy the forces to counter them making the battle easier.
Sentry Guns
To get the best out of your sentry guns have two or more with overlapping firing arcs.