How do I prevent my hydraulic brakes from getting pressed/damaged when the wheels are off? When I was buying my bike, the technician in the shop told me not to press the brakes when the wheels are off (detached), otherwise the brakes will get damaged (I don't know how).

I need to detach the wheels for a few days and I want to clean the bicycle in the meantime. I will be moving it a lot, rotating it upside down, etc. How do I make sure the brakes don't get damaged during this process?
Since bikes are usually shipped to the dealer with the wheels disassembled from the frame, the disc calipers often each have a plastic separator pressed into them that serve to prevent lock-up during shipping. The Hayes company even puts their brand logo on theirs. Since these are removed during assembly, you might be able to ask a repair person at your local shop if they have any extras lying around that you can just have to keep. 

EDIT: Here's a link to a how-to that gives the same advice, plus has some photos of the separator in use... http://www.singletracks.com/blog/mtb-repair/hydraulic-disc-brake-service/