Look at the quilts on display first. Pay attention to the ones you don't like. Study them. Figure out what you don't like about it and make a mental note so you don't do the same thing on your quilts. If allowed take pictures of both your favorites and your least favorites.
Second, do a fast walk by of the vendor booths. That gives you an idea of what everyone is selling. Then go back thru the vendor booths and do your shopping.
Take small samples of the fabrics you are trying to match.
I carry a small notebook that lists all the things I own - like rulers, patterns, odd thread colors (so i don't buy them again), and books. Keeps me from buying duplicates.