Get non-slip rulers. I didn't realize how much a small amount of slippage could multiply over the many squares of a quilt. My first quilt was a disappearing 9 patch made with 5" squares. I think I had to square them down to about 13 1/2" to get them all the same size. I'm sure learning to sew a 1/4 inch seam had something to do with it as well.
For non-stick, I prefer Creative Grids. If you have some kind that's not non-slip, you can use sandpaper dots from the quilt shop. The ladies on the gardenweb quilting forum swear by Nexcare tape from the pharmacy. Invisigrip also works, but it comes off your ruler after using it for a while.
bkay