This entirely depends on the venue of the art show and what sponsors can expect to get out of their sponsorship. If you ask restaurants for food, perhaps offer them a place to set up mini menus and areas for signs to be hung or flyers to be placed.
Your proposal should be very brief. And, it's even better to pitch in person if you are keeping it local in your community. Tell them the date, time and what you need. Offer a variety of sponsorship and make sure to mention the names of your other sponsors to help qualify that they're considering a legitimate business idea.
Will their name appear on your posters? Will their logo be on your website? Facebook invites? In the free advertisement that hopefully you'll get from the local newspaper? Can you guarantee any type of press printed or online?
Here's a hub I wrote a while ago with more info on the subject: