Let me try this again <g>
I have a huge stash, some of it has been donated, some has been given from estate leftovers ... and most of it has come from my "checking out" what was new in the Local Quilt Store ...
One of my friends says that when she goes out of town, as in shop hops, she buys the sales fabrics that are in the neutral range.
Personally, the background fabric is the last thing I have ever thought of, consequently, my "stash" (which is more like a second fabric store - - I am NOT bragging - - I am admitting to be out of control!)
my stash lacks two things - - neutrals and solids!
Perhaps if I had had that one tip ... buy a coordinating fabric ... my stash would be more useful.
On the other hand <g> my scrap quilts most definitely reflect my frenetic buying habits ...
I want you to enjoy quilting, and buying fabrics is part of the fun ... gets the creative joices flowing, but you are wise to have a plan as you start or you will have lots of fabric and STILL have need for more, as others have mentioned.
The idea of joining swaps of either fabric or block swaps or any of the many swaps on this site will further grow your stash ... simply because the swaps give you a target to buy toward.
And, don't forget that you have a quilting personality ... there are just some blocks or styles to which are you drawn ... there may be a holiday or celebration you participate in (mine is Fourth of July and Christmas) ... with those criteria in mind, you would purchase toward that color way.