Every year I try to go to the big annual event held by the Society for Creative Anachronism and I've just returned. I didn't take any pictures this year, partially out of laziness and partially out of repetition. People come from all over the world and hold competitions, display their Medieval style art, sell an amazing variety of things, eat, drink and socialize.
First - go where they want you. If you've been a vendor at a show before and it worked out for you, go back!
Sell what they want! Every show has its own market and how well you fit into it will determine whether you will be successful. At this one, artisans who handcraft authentic items using period techniques are favored. Artisans who create historic type items using modern techniques also get a bit of a boost. There are plenty of people who sell T shirts and bumper stickers but if that's all you're planning on selling you're not likely to win one of the coveted new merchant spots.
Have plenty of inventory! You will not make any sales and you may not be asked back if you don't have enough inventory to cover the length of the event.
Have help! Everyone has to eat and answer nature's call. You are open a given number of hours and if you aren't there you will not do well.
Sweeten the pot! One of the most popular shopping days is Wednesday's Midnight Madness. The merchants start packing up Friday and Wednesday night stay open late to decrease the amount of stuff they're going to have to cart home. Some (the popular ones) offer special deals. Hours will be determined by the show but you can have special coupons for returning customers or on your Facebook page.
Make people want to look for you next year! You've got the most awesome items the show has ever seen but returning customer coupons and careful customer service will make people remember you. Every year I shop based on my memories of the previous year. The merchant who carefully wrapped my pottery and gave me a nice, reuseable, WATERPROOF shopping bag will be my friend next year.
Finally, don't forget to enjoy it!