I have tried the blowdry stuff once, and it ended up on the floor after a few good gusts (old house/windows). I now use 6 mil poly sheeting hung up with a few staples and duct tape. Start by using a staple in both top corners to hold it up, duct tape the edges, then put a staple in each corner so they hold onto the tape as well as the poly. If the window is large, I put a few extra along the top and bottom edges. Put the staples in an inconspicuous spot so the holes aren't noticed later. When the season is over, the duct tape residue can be removed using a citrus based adhesive remover (not as toxic smelling but powerful). Spray it on a rag, then wipe the residue- this will help in not messing up any finished surfaces. The poly can sometimes be found at building supply stores like Lowes. You can also check out environmental supply stores in the yellow pages. It's usually sold in 20' x 100' rolls- average price being around $60-$70- but this will last you quite a few years and will more than pay for itself in reduced heating costs. In rooms that aren't used often, I seal them off with flaps (so I can still access them). If you want any pointers on doing that, please let me know. Guess I should mention I do asbestos abatement so I do this type of thing on a daily basis!