There are a variety of solutions for this. I believe that in the forum archives there are a couple of old threads on the subject. Swampy and I both made wheels for our boats that we attached with bungee cords. They work great although my design is far superior. }(

Swampy took some pics of us fishing last weekend and I laid my wheels across the back of my boat after launching so they wouldn't get ripped off. Here (hopefully) is the pic. Construction was pretty simple: about two feet more PVC than the boat is wide, a steel rod about four inches longer than the boat is wide, two PVC "L" joints, and two wheels that fit the steel rod.

Buzzfish