Careful with the brakes especially front.Sand is great fun and you can ride super fast in it once the technique is learned.Remember when in doubt throttle it out!
Listen to this advise... using the brakes in the sand is a tricky proposition ( that is if you're trying to go fast) I'm not sure what your riding ability is, so you must take that into account with any of the advise offered here, including mine.
EXPERIMENT!!
Try different techniques out as you're riding, see what works best for you.
As been mentioned before, don't brake too hard or too soon. Why? In a safe sand area do a little test. Accelerate up to a turn and let off the gas, observe what your bike does...
Next find an area where you can really lay the bike over... what I mean is the drag your bars in the sand laying over....
In many cases you will find that the faster you go in the sand, the faster you can go in the sand... but you need to hone the skills that will allow you to do that...
Just so you all know I have ridden at quite a few sand tracks, including Croom and Southwick... sand is awesome.
Oh yeah.. the number one tip for going fast in the sand.... be in GREAT shape. Riding fast in sand will wear you down quicker than any other form of riding/racing.
Have fun!