This post from Clark Howard has some good ideas for how to get out of a timeshare -- along with his I-told-you-so Never Buy A Timeshare! LINK
From the article, a few suggestions, once you realize you can't sell it:
Call the timeshare company and offer to give it back to them.
Give it to a charity. (But don't expect a big tax deduction for it.)
List it for $1 on The TUGBBS Bargain Bin. (Don't buy one for a dollar there! You'll be sorry!)