I started out using Mary Ellens Best Press (expensive) then I used the spray starch (flaky and messed up my iron) and now I use Magic Sizing. Love the stuff. It's cheap, clean and works well. Love it for piecing especially if you're working on small, bias edges like I do when I'm working on the Farmer's Wife 6" blocks.
I DO place a piece of muslin or flannel on my ironing board cover when I'm doing a lot of spraying so I can throw that in the washer when it gets yucky. Saves my ironing board cover.