Presales - Learning from the Experts

When a distributor buys a film that hasn't been made yet they are almost blindfolded - they can't see the finished product.  So what determines what they are willing to pay ?

Even though old timers mutter about how pre-sales today aren't anything like the glory days .. for those of us in the indie sector it is still a vital part of film financing.    So I thought I'd ask some experts for some answers on what we need to do to our projects to help pre-sales.