I well remember my early days playing Dungeons & Dragons.
Every adventuring party stocked up on such essentials as iron spikes and ten-foot poles, to better deal with the challenges a dungeon could provide. I don't think we realized until later that the spikes were for securing doors, but we knew right away that the ten-foot poles were used to probe for traps.
The mental picture I have now of our adventuring parties is that of a blind man exploring a dungeon, tapping his (ten-foot) cane in front of him to avoid dangers. That was necessary, though, because in those days traps were used as a deadly tool by the DM. When creating a new character merely meant rolling three six sided dice half a dozen times, the DM took great joy in killing characters.
These days, though, character creation is more of a chore, and it isn't as much fun to have characters you've invested your time and emotions building up killed outright by a trap.
DMs must be more created now to use traps effectively, to increase their players' enjoyment through the use of traps, rather than as a bludgeon against them.
Here are some tips on using traps effectively.
Use Traps As Comic Relief
In a dungeon, rust and rot are going to play havoc with traps. They may misfire, or simply not work. Picture a thief detecting a trap, and working diligently to disarm it by unhooking the twine that's being used as a tripping mechanism. She keeps the twine taut, to avoid triggering the trap, when all of a sudden the twine starts to unravel due to age! She has to think quickly to avoid the trap firing. And if she fails, perhaps the rest of the trap's parts are in similar working order, spewing springs and cogs into the tunnel instead of firing arrows out the hidden slits.
A particularly demented dungeon creator might use traps as a way of annoying invaders, as opposed to killing them. Pie in the face style slapstick, right in your dungeon! Of course, pies won't keep very well in a dungeon, but I'm sure you could come up with any number of noxious substances that would equally annoy.
Use Traps For Dramatic Tension
Your players encounter a trap. Which is more exciting for them?
Option 1: The thief rolls to disarm the trap, fails, and you roll dice to see how much damage she takes. Whoops, too much, she's dead.
Option 2: The thief rolls to disarm the trap, fails, and hears the sound of gears and stone grinding against stone. What's happening? They'd better find out fast!
Instant kill traps just aren't any fun for players, and this includes the sort of trap that might kill a normally healthy character in one blow due to a lucky roll of the dice.
Turn The Tables!
Create a situation where the players will want to trigger traps. Perhaps they've explored a dungeon and discovered an army of goblins set to invade the kingdom. Along the way they've discovered and disarmed a number of lethal looking traps. Now, being chased by goblins, the thief has to reset all these traps to take out as many goblins as possible before they reach the surface. And all at a dead run.
The Golden Rule
The golden rule to remember is that most players aren't into the whole DM versus Players battle. Deadly traps are mostly a weapon in that battle. Your job as a DM is to make sure that your players enjoy playing, and if they die it should be because they've screwed up regularly, not just because they failed a trap disarm check.
Unless, of course, your players really do enjoy the DM versus Player battle. In that case, have at it!