Traps are about a hazardous to the writer of an adventure as they are to the party that encounters them. A trap that is not thought out, out of place, or does not challenge the party appropriately can kill an otherwise excellent adventure. Traps are a small part of a well balanced adventure but their impact can be significant.
Like most things, traps must be used in moderation. An adventure that is full of traps will fail without question. The major problem is that it will require a rogue and having an adventure rely on a single class in order to survive, much less complete, is a major no-no. Two or three traps are an excellent number for a tournament length adventure.
The first thing a trap designer must do is figure out why the trap was placed and what its intended purpose is. For an example of this, the opening sequence of Raiders of the Lost Ark is a great case study. Those traps were designed to either keep people out or keep the idol in the cave. The traps leading up to the idol are designed to scare off, kill or stop raiders. Once the idol was removed from the pedestal, the traps changed their function. The sliding door and boulder are meant to keep the idol in the cave. The sequence of traps was logical and they all served the same goal.
Once the trap has been picked you need to scale it to account for different level parties. The Dungeon Master’s Guide v.3.5 has some excellent ideas and rules for accomplishing this. Unfortunately, these can be taken too far. My personal favorite was scaling a pit trap until it was 200 feet deep. Can you imagine the evil overlord talking to his trap development crew…” Hmmm. My treasure is vast so very powerful people may come for it, better make it 200 feet.” At some point, the pit trap will become ineffective. Scale your traps by combining elements from different types of traps. Add a glyph or darts to assault the poor victim.
The key element to trap creation is creativity. Combining traps in ways that are unexpected will provide your adventure with very memorable moments. I can recall an adventure where my rogue found a trip wire. No big deal there right? I decided to help the rest of my party to cross over the wire by guiding their feet over the wire. I placed the first guy’s foot square on the pit trap. Down he went, tripping the tripwire on the way down. Awkward moment at the time, but I still remember it years later.