Saw the post about "Cheating Sleep". Figure I'd start a sequel. :)
In all seriousness, monogamy is only beneficial for two groups of people:
1) women - they have a biological incentive, since babymaking takes 9 physical months and years of nurture / time investment. a guy who disappears will not provide resources.
2) beta males - to put it in a condescendingly, they're "lucky" to procure any woman.
A few obvious ones-
a) make sure she's in a different social circle. different town / state is best.
b) have a job with a reputation for long hours. "sweetie i have to stay late, last minute meeting."
c) if she accuses on circumstantial evidence, deny deny deny any and all infidelity.
share your stories / personal experience.