I followed another QB members idea and put a bungie cord with paraclips on it along the front of my rails. I get the backing on and batting and have it adjusted on the tight side, then I lay the top down and get it relativiely straight along the top edge. I then use the chanel locks on my machine (if don't have the computer turned on, I use the plastic C clip to hold the carriage wheel in place) I roll the machine along and pin the top edge where it need to be to have the quilt straignt. Then I straighten down both sides and pin, and then move the clips on the cord to line up with vertical lines on the quilt - I have 6 clips so put one near each edge and the others where I can line up.
Then I loosen the quilt 2-3 notches as my machine likes the top on the loose side.
When I role, that front cord often is adequate to use to keep the front square, and I use the clips to keep the sides striaght. I always roll tight, get everything lined up and pinned on the sides, then loose up. Often I have to pull a bit on the upper sides as the quilting pulls everything in and I need to get it back in place if I'm using a computer design.
Since I put on the marking cord, I no longer need to measure anything when I advance the quilt. Makes it a lot easier.