Make more Tiberian Refineries rather than buying more Harvesters. It may take longer, but it will give you more money.
If you are on a map with blue Tiberium, be careful. Blue Tiberium is as flammable as gasoline. This can work to your advantage or it can be your downfall.
Mutian breeding ground:
Play as Nod and create a Clocking Device near or in some Tiberium Make a large square laser fence inside the cloaked area with the Tiberium. Create a Subterranean APC and Infantry. Use the Subterranean APC to put the men in the fenced area. Take out the Subterranean APC and run the men around in the Tiberium until they change into Mutians. The more men you change, the better.
G.D.I. Service Depot:
Do not press S while a unit is moving in or out of the G.D.I. Service Depot, because it will explode.
Unrevealed Terrain message:
Sometimes when you scroll into the black shroud the “Unrevealed Terrain” message will continuously appear at the bottom of the screen. It can happen in any mission, but will usually will occur on slower computers
Build anywhere visible:
This glitch will enable you to build a building anywhere that is flat and that you can see. First, enter the options screen, go to “Keyboard”, go to “Interface”, and scroll down until you see “Scroll North, South, East, and West”. Assign keys on the number pad to those. Construct a building and place it as you usually would, but do not release the mouse. Next, slide it under the sidebar, making sure no part of it becomes red during the process. With the unplaced building under the sidebar, scroll around to a flat area where you want to locate it, and release the mouse. Note: This requires some practice.
To easily get rid of an enemy in skirmish, build wall (one square) and keep building one square. You will notice that you can build them further and further away each time from your base. Do this until they reach an enemy base then build a load of wall around their refinery to cut off their money supply. They will not shoot the wall as it is making no threat to them, but it will keep their harvester locked out. In the end they will just sell their items one by one.