Variety of color, pattern and texture is the key to a useable stash. The quality of the fabric you use will determine the quality of your finished product. Join online swaps to build your stash. Check out online quilting pages for ongoing swaps. Start a swap of your own with a group of friends.
Swaps can center around color, theme or texture.
Stock up on sales at local quilt shops. Don't be fenced in to your favorite colors. Your future quilts will want to be many different things, so you should have lots on hand to supply them. As a growth exercise, encourage yourself to collect some variety of colors that you really dislike. A fat quarter is the minimum for any fabric, but if you really like it you'd better buy at least a half a yard.