These aren't my tips, they come from those inside the business I've talked to over the past week:
1) Once you have all GFEs in hand, have every prospective lender update their rates on the same day. We're still experiencing a volatile market and you'll see movement of .125-.25% daily. You need an apples-to-apples comparison to make an informed decision.
2) Don't discount service. Too many borrower just go for the absolute lowest rate and fees without taking into consideration the likelihood of the loan closing on time and under the terms initially agreed upon. Good rapport with your loan officer will give you some confidence in their ability to deliver at closing. I see all too often situations where the bait and switch is pulled at the closing table or the day before. If you see something that's too good to be true, it probably is.
3) Once you have proceeded with a lender request a copy of your interest rate lock confirmation. This will ensure that your loan was indeed locked under the agreed upon terms. It should be locked for at least 30 days.
4) Check out the lender's BBB ratings. Also, if your state requires loan officers to be licensed like we do in Texas, you should be able to find a website where you can search for your loan officer to make sure they are licensed.