I respectfully disagree with using trash. I've had plenty of experience in our company of people putting newspaper, shredded material, candy wrappers, etc. around hardware. All this, and even packing peanuts...are BAD. The PC can still move around. 75+% of the hardware is damaged for $100+ ever time someone ships PCs in "trash".
Your best bet is to find a box (or buy one from a shipping company) that is made of double-walled corrugated cardboard. This will cost you (based on the average size of a PC) about $5-7.
You'll need 2-3 inches of null space around the ENTIRE device (you can't lay or stand the PC on the bottom, or you could incur some damage). Then you need packing material...preferably antistatic bubble wrap (1/2 to 1 inch bubbles) and it should completely and evenly fill the null space around the asset inside the shipping container. This will run you about another $5-7. Cables or power cords can be placed in the space immediately around the PC.
Ground shipping will be about $18-25. So in total, the lowest it will cost you to GUARANTEE it's not damaged is about $28, but more realistically around $40.
If you work for a company that wants a solution for shipping IT hardware around, my company bought 20 custom built boxes for about $150/box. It sounds expensive, but I bet we've saved tens of thousands by avoiding shipping damage by ordering them. I know this isn't a solution for home, but if you're in a company that continues to have hardware damaged during shipping, check out Benson Box (www.bensonbox.com).