I'm guilty of the "buy it because I love it" mentality. Some of the stuff that I HAD to buy is still sitting there waiting because I can't decide what pattern to use.
Think about what you might want to make. If you expect to make scrappy quilts, buy small amounts, FQs or 1/2 yds. Mostly I buy anywhere from 1/2 to 1 yd. Sometimes that isn't enough for a particular pattern. You might want to look at some patterns and see if there is something you want to build your stash up to make in the future. If you like patterns with large blocks, you might want some larger prints. If you find a large print you love, consider 2-3 yards to use as a border - you can cut lengthwise borders and use the rest for blocks. If I pick a color that might seem hard to match, I try to buy a few coordinating fabrics at the same time. One thing that I use a ton of is marbles or other blenders/tonal fabric. I love Kaufman Fusions. I tend to use very little solids, but would like to start buying some to incorporate into my quilts.
For a quick stash, try a bundle of something...FQs, layer cakes, jelly rolls, charm squares. Just be aware that they are not usually exactly the size stated. It's easier to live with that if you use a pattern that cuts them down a little. But they do give a nice variety of fabrics already coordinated for you.