First you must determine if they are good candidates for a residential short sale. This means that they have no other assets to liquidate so that they can pay the difference between the market sale price and what they owe. Some people may actually be better off trying in a foreclosure or bankruptcy situation. Some lenders will approve the short sale but still come after the seller for the balance.
Each lender has their own "short sale packet" requirement so you should ask your seller to make the initial call and get a list of documents their lender(s) require to apply for a short sale. This "short sale packet" varies from lender to tender but the basics include current pay statements, bank statements, listing agreement, monthlly income/expense statement, hardship letter written by the homeowner (the more desperate, the better), and a fully executed contract of sale (offer). Before the lenders will speak to you, you'll and/or the attorney your client chooses will need to complete a form signed by the seller giving you permission to speak with the lender on their behalf. It is important for the seller and agent to find an attorney who is very experienced in the short sale process and negotiations. As for other documents, you should check with your broker to see what they normally require.
Hope this helps. Good luck. Short sales take a lot of patience and are not for the faint of heart.