"When an option is chosen from two mutually exclusive alternatives, the opportunity cost is the "cost" incurred by not enjoying the benefit associated with the alternative choice."
For example, if you have the choice between sitting on your ass for an hour and making $50 for an hour of work, you should only choose sitting on your ass if you'd be willing to pay $50 for the privilege.
I know this, but I don't live by it, and I really should.