I have a game board made of heavy card stock that doesn't lie flat at the creases, and the sections bow a little. Is there a way to keep it lying flat? Maybe a way to mount it?
The board is from Commands & Colors: Ancients.
I've always just overbent the board at the creases, and it usually helps it lay flatter.
The simple answer is corrugated cardboard. Cheap and effective, but it looks bad from one end.
Another option is to find a larger board from another game and glue your board to it, trimming the edges of the board and re-wrapping them with paper tape.
You could try adhering the card stock to modeling plywood (the <1/8" variety) to add rigidity, but keep the board from being too thick.
I had a similar problem. The board in my case wasn't creased, but it was printed on flimsy card stock. The board was black so I bought a sheet of black foam core from a hobby store (Michaels in my case, but they carry it at Target, Staples, and many others). I sprayed the back of the game board with some spray adhesive and glued it to the foam core board.
You can either cut the foam board to size before gluing or after. I cut it before and it seemed to work okay, although it made lining up the board during gluing pretty critical and difficult. I found a sharp Exacto knife was the best way to cut the board.
You have several options about how to mount it: