i accumulate stash in three ways. the first is that i buy stuff i like. if i want to make something out of it later, i can usually find cooridinates. the second is that i buy enough stuff to make a quilt even though i don't exactly know what i will need of each color. i may not even have a pattern in mind, but i will have all of the fabric if i do find a pattern. the third is just that stash happens. these would be pieces left over from other projects.
i would say don't force what you choose as stash. start with the intention of making something using a pattern that you have. then, if you like something, pick it up. it all depends on how you decide to use it. this is the hardest part of building a stash since there is no pattern and nothing yet to go with it. don't force the process. you will fall into your own pattern of buying and using or buying and hoarding, just like the rest of us.