How do you make an offer on a short sale?

Short sales transactions are different from others in that a contingency is added to the contract whereby the seller's lender must approve a payoff that is less than the balance currently owed by the seller.  In some cases there may be other liens such as taxes, utilities, subordinate liens, HOA, Condo Assn, etc.  All liens must be cleared in order to pass a clean title free of liens and encumberances.