In WA we have more opportunities to enjoy four-wheel-driving on the beach or in the dunes than anywhere else in the country.
To enable you to get the most out of your 4WD on the sand, tyre pressures need to be adjusted to the conditions. As a rule of thumb reducing pressures by half is usually a good place to start, however adjusting them down even further will help considerably in soft conditions - you will be surprised at how much difference a few psi can make.
Having said that, for the majority of 4WDs anything under about 12psi could see a tyre being rolled off the rim in certain circumstances but, if the tide is coming in and the vehicle is buried to the door handles, getting down even lower might be the only option.
Letting the tyres down is the easy bit and there are several accessories on the market that can help. A set of Staun deflators screw on to each Schrader valve and let the tyres down to a preset amount or there's the all-in-one ARB deflator which incorporates a gauge and valve fitting that removes the inner valve then reinstalls it when the desired pressure has been reached.
A match or stick to depress the valve and a tyre gauge to check pressures will do the job just as well.
Personally, I reckon removing the valve completely and counting is as quick and easy as any other method as long as you have a couple of spare valves in the ashtray just in case you drop one in the sand!
Approximately one second equates to 1psi, but obviously keep checking with a tyre gauge as the pressure goes down. Getting the tyres back to somewhere near road pressures isn't quite so easy - you are going to need a compressor of some sort. There are at least 20 different types of compressors on the market ranging from the real cheap ones right up to the engine-driven type.
Generally you get what you pay for but most mid-priced - $300-$400 - 12 volt compressors do a pretty good job.
Pumping up four big tyres from 12psi is a big task for any of these compressors. The best of them will pump plenty of volume and have a half-decent duty cycle. Having two compressors on board isn't a bad idea, that way you can pump the tyres up twice as quickly or still have one to do the job if the other fails.