Before you go, check out the different sewing machine web sites just to get an idea of what you want in a machine. Cruise places like AllBrand and ebay to get an idea of what the machine will cost.
While at the show, don't let anyone pressure you into buying that day unless you are sure that's the machine for you. Most sewing machine vendors will have a "quilt show" special and will honor that special price for a few days after the show -- ask!
Ask about service (where, etc.) and classes (is it free; how many; where)
Test drive the machines that interest you. Take the brochure and write notes on it about what you like and what you dislike.
Go back and test drive again the one you like best.
If you find a machine that you just can't live without -- before you buy -- go take a break, get out of the crowds and look at your notes. Sometimes dealers will sell machines that are in their booths cheaper just so they don't have to take them back with them. Don't be afraid to bargain with them. Sometimes it works, sometimes it doesn't but it doesn't hurt to ask.
And most important of all -- HAVE FUN :!: :!: