I made my own 3 season (which I use 4 seasons, the season rating relates to the thickness of the insulation) and followed a design by Ray Jardine. He has a book out that you could probably find at your local library or they could get it for you. I recommend a zipper-bag rather than his quilt-only design to save weight. Don't forget about insulation from the ground-- 1/2" blue closed cell foam pads are the da bomb.

check out http://www.owfinc.com/ for fabric and insulation.
I used polarguard 3D as insulation and very lightweight breathable ripstop nylon as the shell. Cost me about $45.

As mentioned in other posts you want breathability. I usually carry a small tarp made of parachute type material, silicone impregnated ripstop. I think you can still get seconds of this cheap at noah lamport online, they do have a 5 yard minimum! Remember that "yards" is length. Most fabrics come in 64" widths so 5 yards of this stuff is a ton. I'm still making stuff sacks for my friends as christmas gifts three years later.

The other option to the tarp is to make a bivy bag mentioned by others out of a gortex or gortex-like material on top and somekind of plastic coated nylon on the bottom. I don't like bivy bags though. Tent, tarp or nothing for me.