This stuff here: http://www.atsko.com/products/waterp...d-extreme.html is the business! Works really well, they also do this: http://www.atsko.com/products/waterp...welt-seal.html if the seams are really leaky.
All good stuff from the same company that makes Sno Seal.
FJRider's comment about getting a new fly probably isn't a bad idea either.