+1 on what Cookie said. The herds move around after the shooting starts. I've seen as many as 50 in one weekend; took friends back into the exact same area the following weekend and saw zilch.
Unit 30, which seems to hold the most sheep, is only open during February. The other units with good numbers (29, 31-34, 36 and 37) are open from February to March 15. There are some herds outside of these units such as units 9, 18, and 42 that are huntable year-round, but unless you live there or know someone that does, it's really hard to keep track of the few barbarys because they move around so much.
Invest in some tough all-leather boots and briar shun pants. Chollas, prickly pears, yuccas, and every other conceivable "pokey" seems to abound in the terrain these critters like to inhabit. Plan to put in at least 10 to 15 miles per day. The eye-sight of these beasts is pretty amazing--seems like most every one that I've glassed out to a mile is staring straight back at me. Concentrate your glassing about a half mile + in front of you.
The other thing that I think is more than just coincidence is the number of desert mulies that I've actually seen mixed in and around with herds of barbary. They seem to either both like the same terrain, or actually like hanging around each other. Could be that deer use the sheep for their superior eye sight, while the sheep use the deer for their hearing or sense of smell. Who knows?