If possible, try to get a face-to-face meeting with your lender to discuss your options. If you're having problems meeting your mortgage payments, they may be willing to negotiate a better rate. Here;s a good article on How to Stop Foreclosures http://homebuying.about.com/od/4closureshortsales/qt/011708_
But it you have exhausted your options, here's another article on how to qualify for a short sale http://homebuying.about.com/od/4closureshortsales/qt/112707_
Before a short sale is approved, you have to submit a hardship letter explaining why you need help. Some causes could be loss of a job or job transfer, reduced income, etc.
Whatever you do, don't do it alone as this is a long and tedious process. Get a professional to help you --- a realtor is a good place to start, especially if the realtor has experience in short sales. Do note that in a short sale, the lender assumes the responsibility for selling expenses so that would be a big help as well. And if this is your principal residence, you may qualify under the provisions of Mortgage Debt Forgiveness Act of 2007 so that the difrference between what you owe and what it actually sells for wouldn't be counted as taxable income for you,.
Good luck and good fortune.