The original post of advice for Evil Overlords was an immediate Internet cult classic. See the vastly expanded list here, complete with new tips for excelling as an evil henchmen, or a trooper in a legion of doom, or the Overlord's accountant, or even for the Evil Overlord's beautiful but wicked daughter.
A couple of my favorites:
12. One of my advisors will be an average five-year-old child. Any flaws in my plan that he is able to spot will be corrected before implementation.
15. I will never employ any device with a digital countdown. If I find that such a device is absolutely unavoidable, I will set it to activate when the counter reaches 117 and the hero is just putting his plan into operation.