To expand upon my earlier answer:
Find a tenant-buyer that can't qualify for a loan at this time, but can indeed qualify in 3 years to 10 years time. The time factor is up to you. The longer your tenant is still renting, versus buying, the more time passes allowing for the market to come back as well as your mortgage balance to go down via rent payments. I wouldn't sell a house with negative equity to a tenant-buyer. However, you can sell with a contigency clause, if your time frame is 3 years and not longer, you can put a clause in the contract, "subject to bank approval of a short sale", and the market value of the property at the time of the sale. However, extra time allows for the negative equity to go away and is the better approach.
Negotiate a contract and collect option money. You can get fill-in-the-blank lease option forms online, but you're better off getting them from an attorney. A real estate agent can't make a commission on this type of deal, so you can't market the property as a regular sale, since there is no commission until the sale closes, and that will not be in 3 to 10 years time, depending on the math involved. The contract is sometimes added as an addendum to a standard sales contract. Unless you really know what you're doing, get help with the details of the contract from an attorney The most important thing to remember is that you've got to cover not just the money issues but also who is responsible for what types of repairs and other complications that are bound to come up. The thing to remember is that a tenant-buyer that will eventually own the house, will treat the house much better than a regular tenant, since there is no pride of ownership in a regular tenant, but in a tenant that will eventually own the house, they will treat it more nicely.
Agree on the purchase price of the home, which should be fixed on the lease contract. Your tenant-buyer has the option, but not the obligation to buy. You, the seller, on the other hand, are obligated to sell if you enter into such a contract. You'll be obligated to sell at this price, so you want to make sure it's something you can live with. Ideally, the agreed-upon price should be at least at fair market value and maybe slightly more (especially for lease terms of 3 years or more) to compensate for the convenience to the buyer and for the likely appreciation of the property over the term. You and/or the buyer may want to pay for an appraisal to validate the price. Banks and other lenders will only loan against the appraised value, regardless of the price that you agreed on with the buyer. You may want to simply write the contract that the price is the appraised value, as per 2 separate appraisers, one paid for by seller and one paid for by buyer.
Determine how much option money to collect. Some states and municipalities have laws specifying a maximum amount of option money that can be taken, but in general the initial option money or option fee can be almost any amount. A typical figure is 2-4% of the purchase price. I recommend higher. You will keep this money no matter what. If the lessee decides to buy, the money will be credited toward the down payment or the purchase price, and if the lessee doesn't buy, he or she forfeits the option money to you. Keep in mind that many buyers choose lease options because they can't come up with a big down payment, so don't expect to be able to get a huge amount of initial option money. However, this is exactly the money you need by which you can buy your new house that you're looking to move into.
Decide how much of the lessee's monthly payment will be credited toward the option. Anywhere from 0-100% of the monthly payments can be credited toward the purchase price, although the amount is sometimes subject to state or local laws. In general, the monthly payment will be calculated at fair rental value plus a set amount that will go toward the purchase price. This, like the initial option money, will either be credited toward the down payment or the purchase price or, if the tenant doesn't buy, will be forfeited to you. For example, if your mortgage payment, taxes and insurance are $1700 per month, you can make a rent payment of $1900 per month for your tenant-buyer, with a $1,700 per month as rent, and $200 per month as toward the downpayment, in addition to the option payment you received. In this example, you'd take the $1,700 per month you are getting as rent, and pay your mortgage with it and all expenses, and you'll have to deposit the extra $200 per month in a separate account, preferrably an attorney held escrow account.
Decide on the term of the lease. Lease options typically run anywhere from 12-36 months. Less than 24 months usually doesn't make sense for the buyer, and more than 3 years sometimes doesn't make sense for the seller; but if you are severly upside down, time is your friend, not your enemy.