I have been a lurker in these forums for about a month now. I recently started cutting my own mats and backing boards and I had a problem last night while cutting a foam core backing board. I don't seem to be able to cut it without "clumping" the material or tearing it. The method I used was: mark the line I want to cut with a pencil, use a ruler as a support and cut the board with a utility knife. Does anyone have any tips on how to cut it correctly?

Thanks!
Shesh