Flattening Boards?

Anyone have a good suggestion for flattening boards that have a little warping?
I'd rather them be warped than damaged so nothing too industrial.

I place a piece of plexiglass over them when we play. Usually leaves them flat afterwards.

I "fixed" several tiles but never a game board. Take a towel and place it on the tile, on a flat surface. Iron on LOW LOW LOW. Alternatively, I've put tiles under heavy objects and then left them all day in the sun. That works too.

YMMV.

These are actually tiles. I said board assuming treatment is the same.

I'll try some of these..thanks fellas!