When I have that large of a fabric, I fold it in half, then half again. I then will only iron the section I need and leave the bulk of it folded neatly so it's out of the way.
Pretend it is still on the bolt (hoping this makes it clearer), leave the bulk of it neatly folded and work off the end they would normally cut off at the quilt shop.
All of the above ideas from everyone else sound great for the type of rag quilt you are making, but if you just need a section of it for other quilting needs, just relax, iron what you need and cut.
I have a tremor that will sometimes make it hard to use many of my rulers. I find that my onmigrid rulers that have grid lines on the bottom that help grip the fabric. Glad for my go for lots of my cutting!