I have been chasing the stripers on the Cape Cod National Seashore for about 10 years now. First with a 93 ranger, then a 99 ex Sport and now an 05. I must say that airing down is the most important thing you can do. When I first started to go I would only air down to 25 PSI. After exparamenting with the tire pressures I have found that 12 PSI is the best. Keep in mind that the softer the sand the more you need to air down. Up here at the CCNS the sand is very soft thus the lower pressures. If you go to Florida and see people on Daytona they dont need to air down at all because the sand is so hard.
For the OP: you want to make the tire as wide as possable to keep it "floating" on the top of the sand and not dig in and spin. If you do feel the wheels start to spin, STOP and back up. Be careful and don't risk your safety or your ride... Pay attention to the tides too. I have seen MANY people get stuck doing things they had no business doing... then getting swamped by the incoming surf. I don't even want to know what the salt water does to a frame... If you do get stuck, when you get out fix your holes... We are loosing more and more drivable beach every year to envrionmental groups because people abuse the beaches... So please tread lightly and pack out more than you carry in...