A few questions about short sales

[Minna Reid]

Now I'm no expert with this, but I do get at least a few calls now and then from people who are behind, and need out. They generally owe close to 100%. Not knowing much about how to pull off a short sale, I generally steer them in some other direction. But maybe I'm losing business. So maybe I should educate myself.
What is the general criteria for a bank to agree to a short sale?
What kind of discount can I expect may fly? can I pull off 30%+?
How difficult is the enitire process? does it work often?