We have our auction in March and we have already started receiving donations. Just make sure you have a secure area to store your items. We usually go onto the companies websites and look for the "contact us" or donations link. We typically email a company first to save on postage. Many times they will email you back to tell you what you need to send in. Good Luck.
One more thing, make sure you have insurance coverage (you should have it anyway to cover all your events unless you are covered under your school). One year we had to keep all our items at our homes (no storage space in the schools) and we wanted to be covered just in case one of us had a fire, broken water pipes, etc. Fortunately, we didn't need it but you never know.
I'm not looking to send out tons of letters to a lot of companies, but there are a few select ones I'd like to contact. What department do you address your letters to? How much time do you allow for responses?