Also, remind yourself that traps are not your babies, and PCs will often come up with ways disable or avoid them that you never planned for. Let it happen. Never succumb to the desire to find some way to make their solution not work, just so you can see your trap go off.
Traps should not be DM Gotcha! moments. While it makes sense that the inventor of a trap would take great measures to conceal them, the PCs need a fair shot at avoiding, disabling, or otherwise mitigating their harmful effects. How you define a fair shot is up to you, but one of the most useful narrative devices is telegraphing the danger. This telegraphing can take the form of physical objects ("You see a body lying on the ground ahead of you. It appears to have been stabbed three times through the side." There's a pressure plate that triggers some spears that spring out of the wall) or you could just tell the player that his PC's spidey sense is going crazy. I've openly told players that when I say, "Are you sure you want to do that," it is the narrative equivalent of their sixth sense telling them something.
With or without telegraphing, the PC needs a fair shot at mitigating the worst of the effects when a trap inevitably goes off. This usually takes the form of a saving throw. You don't just inflict automatic damage on the guy who tripped the acid trap, or send a guy plummeting into the spike pit. You give him a saving throw to take half of a predetermined amount of damage, or to let him grab the edge of the pit.
All this is to say that while some of the above ideas sound ingenious, and even reasonable security measures for a powerful person wanting to protect his lair, you still have to ask yourself if they're the sort of obstacles that will be fun for the PCs. Remember that players prefer to succeed, and automatic failure is one of the dirtiest tools in a DMs kit. If you're planning on a particularly devious trap, make sure to provide meaningful (not necessarily obvious) countermeasures while you're at it.