I have a folder on my computer of ideas, and a couple of binders of ideas.
When I buy fabric for an idea, I will print out or cut out the idea or pattern and put it with that fabric in one of my many bins.
My goal for 2011 is to actually make some of the "ideas" that are in my bins and clear them out somewhat.
It's not really a system - but I have a feeling that will change this year as I work through what I have.