I'm with the cash only camp. Each month I put my quilting budget in the envelope and when it is gone it's gone. Ideally, you don't spend money from other envelopes on quilting (I have been guilty of taking from the entertainment budget).....hmmmm quilting counts as entertainment right?
It also helps to only buy what you need for the project at hand. I don't like using july money on a project that I'm not going to start until the fall. Remember their will always be another sale.