Controlling air units
Note: This trick works for both the GDI and NOD. To control air units in midair, simply give the helicopter a team number. This allows you to give multi strikes without reloading.
Protect units
If you want to protect a certain units (for example, an ore truck) select a vehicle then hold [Ctrl] + [Alt] then click on the unit you wish to protect. It will follow it around for protection.
Wall tricks
This trick requires a Construction Yard, a high enough tech level to at least build Sandbags, and decent amount of money (depending on where your base is located). Build Sandbags/Fences/Concrete Walls to close up entrances to your base. Make sure the walls are not too close to your base, because the enemy will just shoot over it. The enemy AI is too dumb to know that they can "tear" down walls and they will just stop when they get to the wall.
More Tiberium
Build a "wall" around Tiberium when playing against the CPU. Make sure to also close up the entrances to your base. Then, press R to display the surrender option. Do not press anything else and wait for some length of time. Then, choose the "Cancel" option. The Tiberium amount should have increased on the map. Note: make sure the Tiberium is "regenerateable".
Train free civilian
Build a silo, then sell it. one civilian will appear.
Destroying the Temple of NOD
Surprisingly, the CPU NOD usually keeps its Temple in a small second base, apart from its main base. This base is also usually very poorly defended. There will usually be only one Obelisk guarding it, however they also usually fill it with SAM sites making an air attack futile. Also, the CPU will not use the Nuclear Missile until the Temple is attacked, and for good reason -- there is only one nuke attack allowed per game per player. First, build a second base large enough that you can still fight if your first one, or it, is destroyed. Then, get a large army and mass it outside the base close, but far enough away to be out of range of the enemy's Obelisk. Get an Ion Cannon shot ready and use it to attack the Temple. It will fire its nuke at one of your bases, destroying it. However, now they have used up their only nuke and you do not have to worry about it anymore. Order your army to swarm the base, first taking out Obelisk, then any Turrets, then everything else except for one building. Take over the building, barricade the base with walls, then build walls or buildings over where they had buildings so they cannot rebuild them.
How the CPU cheats
The CPU cheats in the following ways:
It starts with more money.
It can rebuild their buildings even when they have no buildings nearby.
It can rebuild their Construction Yard without an MCV.
You can easily get around the last two, however, by taking out all their Construction Yards. With no Construction Yards, they cannot rebuild without an MCV. Destroy all their Airstrips and/or War Factories to keep them from getting more MCVs.