I've had loaned people 14 times in the 2 months that I have been lending, so I think I qualify as a "pro lender".
1. Read the stickies, and get yourself familiar with those threads.
2. I'm going against what the people so far have told you, don't only take loans with a collateral. From my 2 months in the lending business, most people with good collateral tend to go to the more trusted lenders (zarzab), so not many people will come your way.
3. Research! Before lending, look at their profile, trust, posts. Use google and search (USERNAME) loan onsite:bitcointalk.org.
This way, you can see if they made any public loan requests. Never lend to someone if they have an active loan already.
4. Always ask for a signed message. Even if they are not giving their account as collateral, if their account was hacked, chances are they won't repay.
Use the seclog too to check for password changes.
5. Check the
Known alts of anyone: User generated thread and take a look at their alt accounts (If they have any). Recently, an alt account of someone else who had an active loan came to me for a loan. Duplicate loans are not good.
6. Don't take accounts that aren't bitcointalk accounts, or gift cards or anything you can recover by claiming it's hacked.
7. Taking altcoins? Research the alt coin someone offers you. If there is a small order book, or if the priced dropped a lot, or if it's a scam coin, it's not good.
8. Trust your gut feeling! If a loan seems suspicious, don't provide the loan.
9. Accept escrow to hold the collateral.
10. Always take collateral before sending the loan funds.
11. Decide on what profit you want. Want more BTC? Take loans in BTC amounts. Want more $? Take loans in dollar values.
Make sure to have a term where the repayment amount must be higher than the loan BTC amount if you are giving loans in dollar amounts so you won't be in the negatives loan-wise.
Try visiting other lender's threads and see which kinds of loans they accept/decline and their TOS.
Make sure to have a policy on late fees.
All I could think of, will update if I think of anything else.