Good for you for actually going for some real cowboy clothes. I've been to many CAS matches and for some reason the popular conclusion seems to be that the cleaner, brighter, and more neatly pressed something is, the more authentic it is. Baffles me, I tell ya...
2126 gave a good tip on wearing them for yard work. You could try lightly burying them in top soil for a few days at a time. And honestly, when it comes down to it...just find a nice patch of concrete and lay them out in the sun for about 6 days. That will just about do it in and of itself.
My personal recomendation would be wearing them in the shower until soaking wet, and then wearing them till dry out in the sun (and preferably, rubbing some dirt in during the process).
Ocean water really tears stuff up nicely too, if you are near the coast. Wear them out in the water (and look a bit goofy in the process) and either lie on the beach and wear them dry, or go home and throw them in the sun.
You could also try using bleach, sandpaper etc. But I figure if they are real clothes, you're better off using "real" weathering methods - i.e., just wear them out.