So the "trick" to finding these free campsites should be clear to you by now:
When searching for places to camp, the term to use is "Dispersed Camping".
It's easy enough when you arrive at a new location to stop by the BLM or National Forest Ranger's Office and ask about dispersed camping and if they have maps of the sites. Often you can even find downloadable maps and directions on the website, but they really vary in how helpful they are, so a quick call or visit to the ranger's office will often help you out more than anything. (Forest Service employees are usually really nice -- they're just fellow nature geeks who are fortunate enough to spend they day driving a 4x4 around the wilderness!)
The same applies to BLM -- though unlike the Forest Service, their job seems to be more oriented in the direction of regulating shooting and off-roading events. The best way to find out the specifics of BLM land in your destination area is to search http://www.blm.gov for the state and local region and request information from the local offices.
If you prefer a more do-it-yourself approach, you can also purchase BLM maps that cover a whole state, or zoomed-in topographic versions if you want to scope out potential camping sites from a birds-eye view.
(You can also find other cool information like mining claims by searching the site or Google.)
Also keep in mind...
The other rules of dispersed camping are the usual ones that any mindful camper follows: 1) Always pack out your trash. 2) Avoid camping within 100 feet of springs, so that water is accessible to wildlife. 3) Don't leave campfires unattended.
Other good guidelines to follow are:
- Minimize your ecological impact. Choose to camp at a previously used site, if possible, and stay on existing travel routes rather than trample existing vegetation. Don't make new fire rings. Just clean out the old one. Use already-dead wood for the campfire. Living trees are what makes campsites appealing, so leave them be.
- Be nice to future campers. Don't ruin the fire ring by adding cans and bottles or throwing dirt onto it. The next person will just have to clean it up.
- Leave no trace. Use a fire pan to contain your fire and easily remove all evidence you were there once you're done. Bury your human waste 6-12 inches deep, away from waterways and campsites. Pack out your trash and pay forward a few good deeds by picking up a bit more.
UPDATE: Over the past months full-timing in the Southwest USA in my home-built off grid truck camper, I've personally gotten A TON of great leads on FreeCampsites.net. But over time I've noticed that the sites listed tend towards the "standard" sites that anyone can find, if you just go to the local forest ranger's office. What you won't tend to find listed there are THE BEST Free Camping Sites in an area, simply because regulars aren't likely to advertise their favorite boondocking camps for the whole internet!
If you're interested in a list of personal Top Free Campsites in AZ, NM, TX, UT, CA, and NV -- including favorite destinations like the Grand Canyon, Zion, and Sedona -- you'll want to check out this collection of Free Campsite guides from Frugal Shunpikers: