I was quite impressed by the ideas in this video.
http://www.youtube.com/watch?v=FgCUc_aUn...mp;feature=plcp

This is a hammock specific tarp, but the same techniques and materials can be used for flat tarps and shaped tarp shelters (hexamid type eg). Use plastic drop cloth LDPE or polychryo and reinforced mail strapping tape or high test (2120?) 3M duct tape as the stres line reinforcement. There are multiple examples on you tube if you look carefully. Many people use Tyvec and tyvek tape.