Playing a rogue can present a challenge for those who haven't played it in its current form. Following are tips based on your style of play.
In most parties, many of your comrades will already know where the traps are located and there is little use for a scout. They will simply say, "There is a trap ahead, " or simply run through it. Traps at lower levels tend to be more annoyance than concern. Do not be disheartened! Even if your party runs through the trap, you can still disable it and give the group an experience bonus!
If you are playing with a group who does not know the quest, or are playing some of the higher level quests with random trap locations, you may want to scout ahead. Whoever has the best Spot skill should be out in front. This will usually be you, but sometimes there will be a ranger or another rogue with a higher Spot skill. In this case, you should let the other guy lead. Lastly, share anything in DM text prefaced with "(Spot)" or "(Listen)" if you think it'll benefit the party.
One of the best reasons to play a rogue is the ridiculous amount of damage you can do while sneak attacking. A 20th level rogue with the Rogue Deadly Shadow capstone enhancement and three tiers of the Rogue Assassin prestige enhancement line will do an additional 18d6 (average of 63) additional damage per hit while sneak attacking. Even lower level rogues will do a very significant amount of sneak attack damage. The key to dealing sneak attack is to make sure the monster isn't trying to attack you. This is otherwise called monster aggro, and you do not want it. If a monster is trying to attack you, you will not get sneak attack damage when you attack the monster. This negates the benefit of being a rogue.
Fortification will negate your sneak attacks, but this is typically only found on boss monsters. The Rogue feat Opportunist will help bypass monster fortification. The feats Sunder and Improved Sunder will also help negate monster fortification for all players attacking the monster. The item enchantments Destruction and Improved Destruction will accomplish this same end.
Most monsters without vital body parts will also be immune to your sneak attacks. This includes slimes, plants, undead, and constructs. There is not much that can be done to get around this. Rogues simply aren't as good at killing these types of monsters as some of the other classes are.
Sneaking around with high Move silently and Hide skills will prevent monsters from seeing and hearing you, thus ensuring that your first hit will be a sneak attack. This is your only option while soloing. Soloing as a rogue is difficult unless your goal is to sneak past most of the monsters, assassinating the few you simply must kill. At lower levels against critical targets (sentries or casters), try hard to be out of your target's view in case your first attack misses: you may also wish to open with a Trip instead of a regular attack, as a failure to trip usually won't alert your target. With two weapons, it's possible with the right positioning to simultaneously sneak attack two enemies close together, one with each hand.
When in a party, you should hold your fire until someone beefier than you hits the monster first. Once the monster is occupied with another party member, run up behind it and swing away. If the monster turns around and begins fighting you, block and use diplomacy to shed the monster's aggro. DO NOT RUN AWAY. The monster will chase you and your party members will chase the monster. If you've ever seen an episode of Benny Hill, you have an idea of where this situation is going. Once the monster realizes that you are harmless (because you are blocking and not hitting it anymore), it will turn back around and begin fighting the beefier guy again. Then you can stop blocking and swing away.
In a party facing swarms of weak enemies (such as kobolds), you may actually wish to consider readying a two-handed weapon (such as a quarterstaff for pure rogues), as glancing blows qualify for sneak attack damage, allowing you to potentially get fatal sneak attacks off at three or more enemies simultaneously.
There is little reason to go ranged unless you are sporting a repeating crossbow and have the Rogue Mechanic Prestige Enhancement line trained up. Rangers are made for long bows, you are not.
Update 25 revamped the Rogue Mechanic Tree, making it attractive again. Great Crossbow enhancements are superior to Heavy or Light (Repeating) Crossbows. Unfortunately only a scarce amount of named weapons are available in game, with Crafting some good weapons can be created (GS / LGS / CC / TF / ToEE).
In a fight, if you are pinging away with a mechanical crossbow, hold your fire until a tougher comrade has drawn the attention of a monster. Once the monster has been engaged by your teammate, you can begin firing. You never want to be the first person to attack a target. This negates your sneak attack damage immediately, and the monster will begin chasing you. You will inevitably run away because you are squishy and your party will yell at you to stop moving so they can kill the kobold that is causing all the ruckuss.
With gear, however, you can initiate attacks. Stacking a Deception Item (I like Backstabber's Gloves), an Improved Deception Item (I like Ring of Lies), and a weapon with (another) Deception, Crippling, or Nightshade Venom can do a lot to keep your target receiving sneak attack damage when any of those procs hit. Other gear to consider are the Quiver of Poison, Wind Howler Bracers, and bows with the Radiance (green steel, primarily), Constitution Poison, Virulent, Wounding/Puncturing, and/or Paralyzing procs on them. For trash, Constitution Poison, Virulent and Wounding/Puncturing can create the helpless effect and turn on sneak attack damage quickly with a repeating bow. Paralyzing doesn't make them helpless, but keeps them from hitting you while you wait for deception/improved deception to proc. Radiance creates blindness on critical hits which also turns on sneak attack. Nightshade venom induces sleep, a helpless state and turns on 150% bonus damage.
Be sure to stay within 30 feet of your target in order to keep your sneak attack bonus (15 additional feet with the Rogue Mechanic first core, farther if you cross-class Ranger and take Deepwood Stalker Cores). If you stop seeing the large sneak attack damage bonus, get closer. Sneak attack is going to be the bulk of your damage. There is little to no reason to fire at anything if you aren't getting that sneak attack bonus in close combat. However, you can range pretty well if you're built for it, have a high INT, a good bow, etc.
Because you will likely do a great deal of damage in a short amount of time, you will draw monster aggro. This is why you should wait until your friend has had a chance to land a couple swings before you fire. If you do manage to take aggro away from the meatier characters in the group, you should stand still, block, and possibly use Diplomacy to convince the monster attacking you that you are harmless and that he should return to attacking your friend or throw your Rogue Mechanic SLA's to slow them down, reposition, and keep firing. Additionally, deception procs can help you lose aggro as well.
Do not try to tank anything but fire elementals.
When you attack, try to target enemy casters first. In most cases, if you have a decent DEX (for evasion) and resistance item (for saves), you can take enemy caster aggro better than the melees and do it at long range while closing. Chopping down the "squishy" enemy casters at long (or short range) makes life easier for everyone else, let's you contribute without trying to tank enemy melee, and often gives you something to do while waiting for your friends to take a couple of hits at the melees. Paralyzing, deception, and the others also tends to work very well on arcanes with low Fortitude and Will saves. You will want either Improved Precise Shot feat or Precise Shot feat or a clear line of sight if you want to be able to target casters in the back.
Pulling with Bluff
When soloing, you'll often want to simply bypass groups of monsters or have a hireling charge them while you flank. For the patient types, it's also possible to slowly pare down a group using Bluff at long ranges, drawing one enemy after another towards you. Unlike the combat use of Bluff, by the time the enemy arrives it'll be hostile, but should be alone, out of its comrades' hearing, and if it can't spot you will be initially drawn to the spot where you used the skill and thus vulnerable to a sneak attack if you've repositioned. This tactic is less valuable against primary ranged attackers, who will usually stop short and pound the targeted spot or you with their ranged attack. When doing this, remember the effective range of Bluff is shortest-distance and that the skill does not require line of sight; you can sneak up to target an enemy and then retreat around corners to pull.
Depending on the trap, you can also Bluff enemies into active traps to kill them as well. Gwylan's Stand is a classic practice area for this tactic.
Uses of Tumbling
Tumbling is used in the same situations as blocking: to avoid a special attack or move through projectile fire. Its disadvantage is that it's counterproductive without open space to tumble into. Its advantage is reaction time: you can cancel a regular attack into a tumble, while blocking requires waiting until your attack animation finishes. It can be useful for moving through combat in that it guarantees you'll be stationary whenever attacking (attacks while moving carry a to-hit penalty), and with sufficient points ends up similarly good for kiting. Tumbling also isn't slowed down by being in the depth of water which prevents running. Lastly, as it constitutes a way to rapidly cover a precise distance it can be marginally handy when moving through a trap becomes necessary.