been there, fixed that
superglue. you can still get the saddle loose later by giving it a few whacks after loosening the bolt.
nothing inherently wrong with single bolt seatposts. the problem lies in the shape of the cradles. Most singlebolt posts use a 6mm bolt, so if you hang on to old junker posts you can play around with swapping cradles. Often just replacing the upper cradle with another one that's better shaped to hold the rails will cure the problem. The old Laprade post has good cradles that don't let the saddle slip back. So does Campy super record, which also has probably the strongest single bolt ever used in such a seatpost, with a big honking 13mm head you can really reef on. I retrofitted my american classic post on a road bike with the bolt and upper cradle from a super record post just because I didn't like the looks of the wimpy little bolt american classic uses.