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.

Protect units:
If you want to protect a certain units (for example, an ore truck) select a vehicle then hold [Ctrl] + [Al
then click on the unit you wish to protect. It will follow it around for protection.

Train free civilian:
Build a silo, then sell it. one civilian will appear.

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".


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.