Tip #1 - arrive before dusk to gather driftwood. Look well above the normal high tide line (e.g. in the marsh grasses) for wood because the usual to-fro of the tides will have drawn back most driftwood from the regular tide line. (Note that by the end of the summer you may have to bring some of your own woodstove wood because the driftwood might be quite picked over.)
Tip #2 - look for a couple of large driftwood trees to use as benches. At commonly frequented beaches, these will often already be set in a V or U around a makeshift pit. (BTW it is considered a major faux pas to use driftwood tree trunks as fire wood!!)
Tip #3 - it is not necessary to build one of those cute oval fire pits surrounded by large rocks; this will immediately give you away as a city camper. You are on a stone beach - there is nothing nearby to catch fire! (Exception: if you have little kids with you, you may want to put some sort of visual barrier around the pit).
Tip #4 - determine if the tide is coming in or out. If it's on its way in and you build your fire half way down the beach you may not have time to roast a marshmallow. Instead, I'd suggest building your fire just below the anticipated high tide line. If the tide is in or just heading out when you build your fire, build it on the high side of tide line or you'll end up sitting on wet sand.
Here's a photo of our first beach fire of the season (last night), which leads me to my final tip:
Tip #5 - train your dog to fetch more driftwood while you cook your s'mores. You can see our yellow lab in action just behind the blaze.