Letting some air out of your tires before venturing into the sand is a well known basic technique which increases the surface area of your tires, spreading the car's load, helping to prevent you from getting stuck.

However, let too much air out and you could be at risk of having a tire pop off the rim.

This is usually not a complete disaster, as long as the tire has remained intact, you were travelling at a relatively low speed and the car is on level ground. It's likely that you initially lowered the tire pressure after a long run down the highway, when the tires and air inside were hot. A reduction in temperature will lower the tire pressure, so it is always worth checking your tire pressure first thing in the morning or after the car has been stationary for some time.

Tires which have been driven on will naturally heat up due to friction and the higher temperatures of sand and environment during the day, so unless the tires are very low, it is likely that the pressure will rise sufficiently once you have been underway again for some time.

Useful video:
http://www.youtube.com/watch?v=-9UNFgMXSYQ

We're currently running Cooper STT's on the silver Ford F150.  We usually lower these to about 15 psi on sandy tracks, but we have gone down to 10-12psi without any problems.  Rumour has it that you can run these Coopers on about 6psi if it gets really tricky.

The stock Michelin tires on the white Ford F150 have come off a couple of times on sharp turns at about 12-14 psi, so we try and keep them above this level whenever possible.

We've had stock LR3 tires down to 12-15psi without problems. 

You need to be really careful if you head out on any low profile street tires - we've had group members who lowered their low profile BMW X5 tires to 18psi and ended up shredding the whole set.  They're just not made for it!

Harry Lewllyn from eco4wd.com and 4x4now.com has written a pretty detailed article explaining the physics behind letting your tire pressure down on soft terrain. 

It's a bit technical in parts, but good if you really want to get down to the details:  http://www.4x4now.com/sfjun96.htm 

We have some great tire deflators in stock at the moment from Trailhead:

http://www.blingmytruck.com/products/520-trailhead-automatic-tire-deflators.aspx

It comes with a very useful chart showing how low you can take your tire pressure, depending on the tire type and vehicle weight.