DnD 3.5 - Player Resources - Practical Guide to Dungeoneering
Dungeoneering is more than a Knowledge skill; it's a way of life. DnD includes Dungeons in its name, and "Dungeon Crawl" games are a long-standing and successful genre. The dungeon is among the most iconic and popular adventure locations, and for good reason. No other location conveys such a crushing sense of danger and suspense.
Rule #1: Don't Split the Party.
A party of adventurers is a cohesive whole, greater than the sum of its parts. Together you can overcome great challenges and defeat mighty foes. Your skills complement and supplement each other. Splitting your party reduces your ability to respond to challenges, and vastly weakens you. Many adventuring horror stories start by splitting the party.
If you are doing anything relevant to the plot of the adventure, it should be done while the whole party is preasant. Any situation has the potential to turn violent, and without your whole party present things can go back quickly.
Even if you are scouting ahead of the party, you should remain within line of sight as much as possible, and you should always have a means to alert your party when things get too dangerous.
Keep a Map
Appoint someone in the party to be the party's cartographer. Get a stack of graph paper, and map out the dungeon as you go. Mark interesting land-marks, encountered enemies, and important reminders. Keep an eye out for locations which you can fortify temporarily in case you need to rest. Your GM may like your characters to keep a matching in-game map, which allows him to affect your map in and out of the game.
Cheap and easy to find, chalk is the magic marker of the adventuring world. Mark the walls or floor of places which you have explored. In the event that you become lost or separated it's helpful to know which areas have already been explored and cleared of enemies. If your map is lost or damaged, chalk markings may be your only recourse in a confusing dungeon.
Forewarned is Forarmed
The importance of divination spells absolutely cannot be overstated. Even spells as simple as detect magic can provide information which might save your life. If you are out combat, one of your party's spellcasters should be running detect magic, and the party should stop any time that they encounter magic auras in order to investigate further. Unfortunately, detect X spells are blocked by sufficiently thick material or a thin sheet of lead, so at high levels spellcasters like Liches will frequently line their walls with lead to prevent divinations.
More powerful divination spells are great for safely scouting dungeons. Clairvoyance lets you peek into rooms, and arcane eye can let you scout entire dungeons.
Carry a 10-Foot Pole
Divinations can't detect mundane dangers like trap doors, pit traps, or pressure plates. That's when your trusty 10-foot pole comes in. Pull it out, and tap the floor in front of you as you walk. Poke doors and chests before opening them. Poke anything that looks even remotely suspicious.
A rule commonly used on construction sites is "don't stick your hands anywhere that you wouldn't stick your genitals". It's a bit crass, but it works very well in a scenario where literally anything could be lethal.
Watch out for Mimics
Mimics can be anything; not just treasure chests. They take the form of things which you are likely to touch: chests, doors, tables, chairs, beds, etc. If you don't know that something isn't a mimic, poke it with your 10-foot pole to find out.
Search For Traps
Much like Mimics, traps can be anywhere and on anything. Search every door, every passage, every chest, every landmark. Any trap could outright kill you, or at least weaken you enough to be eaten by wandering monsters. In addition to Trapfinding, there are other ways to find traps, including a 10-foot pole.
The Log Rogue
I literally learned this from a stranger on a bus. Carry a cylindrical log, 5 to 10 pounds in weight, and smooth enough to roll well. If you are about to enter a suspicious room or passage, roll the log ahead of you. 5 to 10 pounds is enough to set of most pressure plates and pit traps, and a log of that weight should be large enough to hit any trip wires running near the floor. If the log hits nothing, it's likely that the area is safe (from traps at least). If the log is destroyed, you're only out the cost of a log.
The Summoned Rogue
Summoned creatures are expendable. Summon Monster I can get you a celestial monkey, and monkeys can grasp and turn door knobs. Send your summoned monkey into rooms that you believe are dangerous, and they can trigger traps for you.
Don't forget the loot
Sometimes you're so happy that you defeated the monster that you rest, recover, and depart without collecting their belongings. Killing things and taking their stuff has been a fundamental element of DnD-style games since ODnD, and nothing has changed. Your enemies' belongings will make up the lion's share of your income, so don't do yourself the disservice of leaving it on the ground where you found it.
Be sure to identify any items which might be magical. Some items are cursed, and using them without first identifying them can cause you all sorts of trouble.