And/or you can put an O-ring or rubber washer where the seat tube meets the frame.

If you put say a bit of old inner tube as a 'sleeve' over the clamp area, I would think that it would be best to check that it's not actually *holding* any moisture there occasionally, especially after riding in the rain/snow.

I read about how someone got an old frame where someone had placed a wadded-up rag down the seat tube, presumably to stop water/gunk from getting down to the BB, but the rag had the unfortunate effect of holding moisture at the point where it was wedged in the tube, and led to it rusting out in that area, from the inside.