A few years ago I saw Mike Krausert (Tattoo) give an awesome seminar on sponsorships at at haunt gathering. Back then he talked about how he'd gotten sponsors for Terror on the Fox (the main one was a casino), but I know now he's gotten some major sponsor support for his new event Nightmare New England (Dunkin Donuts, Monster Energy, etc). I'm not sure if he still gives this seminar, but if you could check it out at one of the conventions it would be definitely worth your time.
The main points I got out of it was just what Darkangel said, what do you have to offer the sponsor? Look at it from their point of view, what are they getting out of this partnership? Also, you have to be able to set a value for everything your offering, which is quite difficult to do. Say you're putting their logo on all your posters, what is that worth to you? How about mentioning them at the end of your radio or tv ad? Also, make sure they know how many people will be seeing their logo over the course of the season. Then you have to ask yourself what you want out of it, money? advertising? supplies? If your asking for money, consider that the sponsor is essentially buying advertising and therefore won't pay for something they don't think will be effective. In terms of getting them, cast a wide net. Once you have them, it's very important to TAKE CARE OF THEM! Remember, they are giving you money and can very easily spend money elsewhere, so go out of your way to make them happy.
I myself have had trouble getting sponsors because our event is still small enough that we don't really have enough to bring to the table, but if you have something to offer, you can attract some attention.
Hope that helps,