For example FR-4 is apparently woven glass fiber and epoxy. According to one source "methylene chloride based paint stripper", "lacquer thinner", and heat will all soften epoxy.
I tried the heat thing -- it doesn't work. I clamped a board into my vise and hung a locking pliers on it to bend it but my 1500W heat gun didn't even start to do anything. Sprang right back into flat after it cooled off. Also worth noting that heat would knock off any of the SMD parts on the board.
I'll try a few solvents tomorrow. Probably a flexible thing like cr0sh described but I'm still curious.