When driving on the beach I air down to about 15 PSI and then air back up when leaving the beach. I purchased this air compressor from Amazon:
[ame]http://www.amazon.com/gp/product/B0012WHBSO/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1[/ame]
It works pretty well, takes about 15 minutes total to air up all the tires. I plug it into the 12V jack in the trunk and the cord is long enough that it reaches all four tires from that jack.