Nearly all mustangs are under the management of the BLM. See their website at
http://www.blm.gov/wo/st/en/prog/wild_horse_and_burro.html
Round up methods vary depending upon terrain. Helicopters, other horses, ATVs, water traps, whatever. Goal is to stress the animals as little as possible. Talk to the people in the program. They are very helpful.
There are also some good books and articles out there.
If you've got specific questions, please feel free to PM me.
Best of luck,
Jim Clark-Dawe
Proud owner of a BLM Mustang