I would recommend buying a rain cover for your backpack and just making sure it is elevated off the ground when you leave it over night.

Thanks Andrew. I have a backpack rain cover. Now I need to make sure to keep the straps, belts etc off the ground. (Need to leave my heavy Bibler Ahawhnee 2 behind for my next trip ...)
-Steve

Or take a big trash bag to put your backpack in. Contractor bags are big and burly, but a thinner one would probably work too.

A couple more ounces and you can have a nice solo tent.

If you really want to keep your backpack, boots, etc. inside your tent, then a Bivy is not for you. Look instead a small tent: http://www.backcountry.com/4-season-tents. I prefer a lightweight 2-man tent to keep my gear in. BTW, "2-man tent" can usually be interpreted as "sleeps two very small men who like each other a lot without their gear". Hence, you'll want a 2 man just to fit one person (you) and your gear.