I have a hard time not buying fabric. It is a pleasure for so many senses that I don't seem to mind pulling out the credit card. However, I took a hard look at my stash and had to admit that I am fast approaching the ludicrous level. NOW, the only fabric I am allowed to buy if IFF I need it (and I mean REALLY NEED not want) to complete a UFO, for backing (because I don't have many large pieces).
Last week I got to buy some fabric to complete a border on a Round Robin. It surprised me, but I actually figured out how much fabric the project needed and I managed to buy that much with a tiny fudge factor for shrinkage. Perfect!
You just need to be honest with yourself.