I keep the underside rinsed off well during every wash and after any trips to the beach or snow. Also, typically once a year I get underneath with a wire wheel scrubbing out any rust I can find (usually from scratches) and then painting over with rust converter for good measure. After 10 years the underside looks great with no rust. BUT I am in interior california and have only had it on the beach once - so pretty low rust environment for me. So you probably should step it up a notch and follow some of the other suggestions given. I've heard good things about fluid film as well.
07 Jeep Green Rubi Unlimited, auto
Mods: Tuffy Cubby Storage Cover, Rugged Ridge grab handles, Shrockworks Sliders, Rock Hard Engine / Gas Tank Skids, Rhino-lined tub, Kilby Evap Can Relocation Kit, 16x8 Centerline Wilderness Series - Falcon Rims, 285/75/16 Interco SSRs, OME 2" HD Lift, PSC Diff Covers, Superwinch EPi 9.0, BulldogFab Rear Bumper, RedrockFab Front Bumper and skid