I also use one of those 1/4" or 1/8" rulers that are made for paper piecing and just plain old copier paper. That works well for me. However, if I have not been PPing for a while, I always seem to make a mistake about 4 or 5 pieces in. Once I get that out of my system, I am good to go for the rest of the time. I have gotten pretty good at "unsewing" one of these seams by using my very small rotary cutter. You know what they say: "Practice makes perfect". Also I will often use scotch tape to "mend" the paper seam where the mistake was.