Sandbagging
The game's AI fails to recognize walls of any kind as a target, which can be exploited in a variety of ways. Apart from using cheap sandbag walls to protect your own base, you can wall off important passages, or even build a wall around the enemy base, locking his Harvester in or out as well as all of his units. You can then take your time to build up whatever army you need to clean up easily while the enemy can't do anything except watch from the other side of a wall it can't destroy.
Stealth units
For this cheat you must be NOD,and be in a multiplayer practise (i have not tryed with online games or LAN)and get about 500 $, in the Character Buy Screen, become a Black Hand (the most right one because its the cheapest) and kill one person, but dont be hurt. Then you must go to the Character Buy Screen again and become any NOD unit you like. If you did it right the person you are now is stealth just like a Black Hand unit.
Surviving air strikes the easy way
When you're playing the Nod campaign, the AI GDI player will always send its air strikes against your northernmost unit or building, whatever it is. Rather than building expensive SAM sites, of which you'll need several to stop the A-10s before they do any damage, simply place one cheap minigunner a safe distance north of your base, and the poor guy will draw the next air strike to him.