My love of Billy Gibbons is well known around these parts, and one of my favorite ZZ Top tunes is "Just Got Paid" from 1972's Rio Grande Mud record. The original is in open E tuning with Billy using a slide on certain parts, but I've kept this reworked riff in standard tuning for simplicity. The main germ of the riff is pedaling back and forth on the low E string to give a thick, heavy, percussive sound.
I like to play pretty hard when I play a riff like this, giving me a good attack and keeping it aggressive. To bring down the dynamics I just palm mute but keep the intensity of the picking the same—we don't want to lose that driving groove. As you can see I'm also alternate picking the whole thing, and for fun threw in Jake E. Lee's faux delay trick that never fails to get people's attention. You could be playing the most difficult stuff in the world and nobody may notice, but do the Lee faux delay and everyone will ask you about it.