Here are a few tips for you from what I've learned during my stint lending money on Prosper .com. Using these tips should help you make the most of your lending experience and maximize your profits:
1) Only loan to those borrowers with a C credit rating or better.
2) Only loan to those borrowers with 0 delinquencies in their credit history.
3) Try to fund only those loans that are automatically funded.
4) Aim for a interest rate of at least 20% on each loan.
If you look at this website, Eric's Credit Community, you can see that as of August 23rd, I am #267 of all Prosper lenders, based on my ROI to Risk ratio. So I must be doing something right.