Via Kottke I found an extremely useful website: How to be a Successful Evil Overlord. The author notices something that I also have noticed but he (unlike me) puts some thought into a solution:
I've noticed that no matter whether they are barbarian lords, deranged wizards, mad scientists, or alien invaders, they always seem to make the same basic mistakes every single time. With that in mind, allow me to present...Didn't I tell you this was a very useful site? Here are my favorites:
The Top 100 Things I'd Do If I Ever Became An Evil Overlord
21. I will hire a fashion designer to create original uniforms for my Legion of Terror, as opposed to some cheap knock-offs that make them look like Nazi stormtroopers, Roman foot soldiers, or savage Mongol hordes. All were eventually defeated and I want my troops to have a more positive mind-set.
23. I will keep a special cache of low-tech weapons and train my troops in their use. That way -- even if the heroes manage to neutralize my power generator and/or render the standard-issue energy weapons useless -- my troops will not be overrun by a handful of savages armed with spears and rocks.
50. My main computers will have their own special operating system that will be completely incompatible with standard IBM and Macintosh powerbooks.
52. I will hire a team of board-certified architects and surveyors to examine my castle and inform me of any secret passages and abandoned tunnels that I might not know about.
65. If I must have a computer system with publicly available terminals, the maps they display will have a room clearly marked as the Main Control Room. That room will be the Execution Chamber. The actual main control room will be marked as Sewage Overflow Containment.
81. If I am fighting with the hero atop a moving platform, have disarmed him, and am about to finish him off and he glances behind me and drops flat, I too will drop flat instead of quizzically turning around to find out what he saw.
Time to go conquer the world ...
