If there's a realty rental agency for that area, I'd start there. For instance, Virginia Beach has 2 main ones for summer rentals, and I'd use either one.
A friend got totally scammed on a Craigslist rental for Va Beach, and I've never forgotten it. If you choose to research on the internet, Trip Advisor and VRBO.com both seem decent. Really though if it's a close destination, I'd just ask my friends for recommendations. Here in Virginia, a lot of folks go to NC or SC beaches, so I've been asking around for recommendations for a trip to North Myrtle Beach we're thinking about. I've gotten some good advice about what realty company, and which condos are decent.
Find out if you need to bring your own towels/sheets/boogie boards, sand toys etc. Find out what the kitchen is equipped with. A lot of places also either have grill rentals, or there are restrictions on grill use. How close is the grocery store/movies/shopping?