I've used both fine tip and regular tip Sharpies, then heat set with a DRY iron. I press both the front and back of the block. Maybe that's redundant, but it makes me feel better LOL!
Tip 1 -- take some pieces of fine grit sandpaper (they come in 8-1/2" x 11" sheets), and have the signer place the block on the sandpaper. This will prevent the block from slipping while they are writing on it.
Tip 2 -- don't stack the blocks until you're sure the ink is dry!
Tip 3 -- I made some "frames" from cardboard to cover the outside edge of the block 1/2" all around the block. That way, no one signed uin that area and none of the message gets lost in the seams.