Put your system together on your desk. If you have the motherboard outside of the case, just use the box the motherboard came in to keep it from damaging the desk. I do recommend wearing a grounding strap though. A little bit of static electricity can fry your entire system.
Do not allow fluids on your desk while building the system. Once the system is put together, never allow fluids above the system.
This is all common sense, but so many people never think about it, and one spilled drink can do hundreds, if not thousands, of dollars in damage.