At each of the BLM areas there is usually a host with a table set up and a flag or two along with a sign reading "Registration". They are usually located close to the road out in front of the camping area. Just stop by and sign in and you will get a sign to put on the rig good for 14 days and it is free.
We like Scadden Wash which is on the south side of I-10 about 2 miles east of town. If you come in from the east on I-10 take the first off ramp and turn left. Take another left at the second stop sign and just follow the road until it curls back parallel to I-10, ahead on the right about half a mile you will see the registration rig. :C
