Specify an overtime rate, even if you think you won't use it.

When setting up costs for a work resource, always enter an overtime rate as well as a standard rate, even if it is at the same rate.  What happens if you allocate a work resource some overtime, perhaps in order to fix a scheduling issue, and the resource's overtime rate is $0.00... ? Budget blow-out potential here, but may be avoided if you can get into this habit.