Basically, sand gets deposited, and removed, from the beach by a series of natural events.
Typically, sand is deposited in the summer, and eroded away during the winter. This has to do with the size, frequency, direction, and power, of the waves at a given time. Summer waves are generally long and slow, and drive the sand from the deeper waters up onto the beach. In winter, the waves are storm-driven, and come in harder, faster, and more frequently, which tends to erode the sand out into the deeper water.
Wind direction will drive waves onto a beach, or if the wind is "off-shore", will flatten out on-coming waves and drive the water out.
Storms can erode a beach. Hurricanes are notorious for this.
Basically, spend A LOT of time at your beach. You might even want to keep a log book of day, wind speed / direction, storms in the area, etc. Look at your beach for things that are planted solidly into the deeper "hard-pan", the clay or rock bottom under the sand. These can be pier pilings, stairways leading to/from the beach, rock formations, old shipwrecks buried deep, etc.
After awhile, you'll be able to tell when a lot of sand has built up on your beach. Hunting in those conditions is tough, as you're only going to get fresh drops, and those are sinking constantly, moving out of range of your detector.
Most saltwater treasure hunters try to work the troughs (if the rip current isn't to strong), look for holes in the sand, "blowouts" (where the rip current returns to deeper water), and during the winter, hope and pray for a good storm to strip the beach of sand, exposing the hard-pan beneath, with all the goodies deposited for years, decades, and sometimes even centuries. Wait until this winter, when the New England hunters start posting their winter finds.