There is only one place in my region to ride in deep sand and it's over 3hrs away so I don't go there much. I really hate sand until I ride in it for about an hour, then I am fast and comfortable for about 30 min, then I'm worn out from the first hr of fighting it. To me, the only way to be fast and smooth in sand is to constantly ride hard and attack it. I usually ride on extreme rocks and technical stuff, so going to sand is the biggest change possible. Speed and agression, with proper technique and no laziness gets it done. You really need to stand 90% of the time as well, which I can't due to a bum leg which is too weak, so when I am going to ride in deep sand I use a very tall seat. I am 6'2", so I mean reallllly tall! So tall I can barely touch the ground. It makes the seated-standing transition much easier, and even when I'm sitting, I can move around better, weight the pegs easier, and slide back and forth. With the very tall seat I can manage to stand maybe 40% of the time, while most of the time in technical terrain I sit about 90%. With a low seat I can barely stand any.
If you are young and strong, forget the tall seat trick, just stand up!
I also like to set up my bike much stiffer than usual. It seems to help keep the bike up on top and floating rather than wallowing. Also, in deep sand is the only time I'm a true power junkie. A very quick 2 stroke is best for me in sand, the lighter and faster the better. In rocks I like a manageable power,soft suspended bike. 20 years go I was about equal speed on a powerful 4t or 2t, but lately the extra weight of a 4t, and the slower power delivery really work against me.
Can you say: KX/CR/YZ/KTM/500 2stroke? My favorite was the KX. It had a good powervalve so it had a somewhat manageable low end with endless top end. Another great bike for sand is an older KTM 380 2 stroke. They were slow turning and stable and had almost all of the power of a 500. I'm sure a GG 300 XC would also be very good.
But the main thing, as for anywhere, PRACTICE! All the setup in the world in no replacement for hard work and dilligence.
Speaking of practice, I read that in years past all of the Euro MX teams, who often had long whooped out sand tracks to ride, made their riders push bikes around the track in the sand! Can you imagine, pushing a bike all the way around a sand track?
I read of one US rider that when he first started riding for one of the Euro teams couldn't even push his bike 1/4 of the way around a sand track. Within 6 months he would push a lap, ride a lap, push-ride for an hour straight for training. Then rest an hour, eat, drink, then do 2 45 min motos with 15mins. rest in between. Now that's training. Scott Summers also pushed his bike for training, and he rode an XR 600!