Depends on what you're aiming for. Are we aiming to kill, cripple, confuse or confine ? That's a major factor. Are we looking for instant death, slow death or guaranteed death regardless of time elapsed ?
Knowing the type of your expected visitors helps too - and that depends on location, time period and available tech level. A pharaoh's tomb will be getting a decidedly different class of visitor to a Bond villain's secret lair.
For a trigger, its always good to play off human nature. Adventurers, for example, are a greedy bunch, so anything set off by grabbing 'teh lootz' will do the trick; or indeed a treasure that kills through prolonged exposure. Spies or thieves will want to infiltrate, so set up pressure triggers in your crawlspaces (be it a ventilation shaft, Jeffries Tube or any other tiresome over-used plot device like that). Because no matter how agile they are, you've got them in a confined space so they won't be dodging whatever you throw at them. And if you get them to enter the confined space through their own volition - for example a supposedly 'secret door' leading directly into said trapped area - the victory is all the sweeter as they only have themselves to blame for entering it.
Got a remote area only accessible by being in good physical condition (i.e. involving much climbing, pit avoidance etc) ? Halfway through try blinding the invader with hot sand or lye if low tech or lasers if high tech. It'll greatly add to the chance that they fall victim to your other more avoidable traps that a fully-sighted individual could avoid.
I'm not even going to start on the subject of poisons. Other than they're a great leveller.
I could say more, but this post is long enough already.
