Been doing that for a while now, and trying to convince friends of mine to join in.
I always liked the idea of the Colonial Marines, so I decided to Cosplay that.
The helmet the marines wear is actually a type of snowboard helmet that you can just get off the internet for about $25.
Have tons of links and info about the actual stuff they used for the costumes on the show, so PM me if you would like me to point you in the right direction.
So say we all.