I had a great rent to own program that we ran successfully for a number of years until the current economic crisis shattered our model. We actually bought homes specifically for tenants to do rent to own.
One thing we did was gave buyers 2-3 years to close. (that way my seller got to do a 1031 tax deferred exchage)
We would typically charge 5% down non refundable to the potential buyer.
We charged about a 20% premium over fair market rents and applied the extra toward buyer's closing costs if they closed and made every payment on schedule. That way, the underwriter could see a two year history of making higher payments and it made it easier to approve a loan even if they buyer had marginal credit.
I have them go try to get a loan approval right up front with a lender that way if they can, they just buy it now, and if not, they have knowledge of what they need to fix over the next two years!