Rotary cutter, mat AND an O'lipfa ruler 24x6. O'lipfa rulers have a lip on the end that butts up to the mat perfectly. The ruler will not move when you cut this way.
www.overstock.com has them for $13 this week I noticed just yesterday.
When you do the search, be sure you put the apostrophe in the name or it won't come up. O'Lipfa rulers