Two things:
1) Do you use the ruler grippers? If not, then I suggest it highly. If you don't want to buy the little sandpaper ones you can cut some out of sandpaper from your house and glue on.
2) When you fold your material before cutting - make sure that it lays flat, no wrinkles what so ever. Line the bottom up on the mat line and sort of flatten out then place ruler and line that up.
Hope this helps.