I had a lot of trouble with this when I first got out of college (a whole whopping year ago! lol) and went from a max of 4 performances of a show to a show with 30+ performances. (Hardly 100+, but still can get mind-numbing after the first two weeks or so.) The theater where I work currently has the SM also act as LBO (and occasionally also SBO, which is an interesting console configuration to say the least!) so it doesn't get as boring as it might, but some shows will always make you sleep nomatter what you're doing.
On shows that require more attention, but still have duller moments, I play the waterbottle game. For some reason doing something with my mouth makes me not zone out, so I keep a filled waterbottle on the floor beside my chair, and during the duller moments I take it out and take a sip every time someone misses a line. You can tell how well your actors are doing by how quickly you run to the bathroom at intermission!
On shows that are really low-impact (ie, The Importance of Being Earnest, a 3 act show in which I had about 6 sound cues and 8 light cues... not very exciting), I embroider, play Diner Dash on my phone (you get to where you can play it without really looking at it), or make chain mail. (I'm a renfest nut when I can escape the theater for five minutes...)
My ASM for most of the past year used to read, which I did for a while, but I find myself zoning out REALLY easily, so I certainly wouldn't do that if I'm the one calling the show!