I took a day-long course in sand driving at Pismo Beach with Tom (tom@4x4training.com). I aired down to 18lbs front and rear and had no issues. I did get stuck in this bowl - you can't tell how steep it is. The SMB behind me started before I got stuck so he tried to go above me, got stuck too, and, as you can see, nearly crashed into my van. He broke at least one tire bead sliding down the hill towards me.
My van is 10,500# and I will echo those who suggest air pressure management as the key. Have the ability to air down and air back up and you can get through any sand I've encountered.
When I first got my van I ran 60 front/80 rear on the road and then 30/40 on dirt roads. But then I lost 3 rear tires - one because I was lazy and didn't air up and drove down the highway in Death Valley - and the other two because I was aired too low and and bounced over rocks which in one case broke my wheel in addition to giving me a flat tire. So I pay more attention to my rear pressure now than before. I actually weigh 4700 front 5800 rear so that extra weight in the rear does make a difference. Still, if I hit soft sand, I have no problem going low (18#).
I got stuck in beach sand another time running street pressure in 2WD. As soon as I aired down enough, I drove right out. Oh, I also drive with two spares now so I can keep driving if I get one flat out in the middle of nowhere.