The last time we had a Melee Academy, I riffed on +Douglas Cole's post on melee positioning, giving suggestions on how to do what he recommended but better. That worked pretty well so I'm going to do it again, but this time I'm going to reinforce what he said.
Doug's advice is simple, but good: stay close to to your allies, compensate for each other's weaknesses, focus your attacks on your enemies to overwhelm them, and protect your allies when they're wounded. Those are all great basic tactical principles. But if you're part of a group in which each person goes off haring by themselves and gets ambushed and surrounded by enemies, you may wonder how you can put those principles in practice.
CommunicateRole-playing games are meant to be social and co-operative experiences, and its strange how people will stop being social in combat. As soon as the GM starts setting up the initiative order, everyone begins assessing the table and planning their own actions, but at most tables, people don't ask each other what to do.
This is a bad habit, but it's an easy to break. Before making any plans, ask the other players what they intend to do. The rogue may want to run forward and stab someone, but the wizard may want to fireball all the enemies clustered together. A few words from the rogue's player or the wizard's player can be enough to stop the confusion.
Be PatientA good step to not haring off alone is to not go haring off at all. A lot of players want their characters to close with the enemy as quickly as possible. In a way, this is reasonable: each player's turn is their time in the spotlight, and it can be hard to resist the temptation to do something that seems proactive. A simple declaration of "I wait here for the enemy to approach" doesn't feel proactive and means giving up the spotlight almost as soon as the player gets it. But if almost every player makes a simple declaration, the turn goes by fast, and the spotlight returns to each player quickly.
The advantage of staying still and waiting for the enemy to approach is the PCs are generally aware of their immediate surroundings and in something approaching a formation at the start of a fight. If they all charge toward the enemy, they move into an area that hasn't been scouted, with unknown potential traps or ambushers. At the same time, unless the players are really good at co-operating, differences in character speed can cause formations to break up.
Another advantage to staying put is that PCs generally have superior ranged options to their enemies. In DF, the PC Scout probably has a higher accuracy, better damage, and a faster rate of fire than most enemies, and the PC Wizard can use all kinds of nasty spells to control the terrain and damage incoming enemies or channel their approach that most enemies don't have. Use those comparative advantages well, by making the enemy to come to the PCs and extending the amount of time that the PCs' ranged specialists can rain doom and destruction on the enemy.
The final advantage to waiting for the enemy, at least in GURPS, is that the Wait maneuver provides a huge tactical advantage when used well. Melee opponents who have to make Move and Attack maneuvers to reach the PCs are already at a disadvantage, and if the PCs can strike the enemy as the enemy moves into reach, things are even better for the PCs. Enemies who slow up as they approach spend more time under the eye of the Wizard and Scout, and have to do it at can't miss ranges.
Make A PlanAfter a group starts communicating with each other, they can move to the next step: making a plans. Plans don't have to be complicated or involved, and they don't need to have a lot of contingencies. A reasonable plan can be made after all the PCs have acted once, and should cover what the PCs intend to do with their next actions. This should be simple stuff: "The Scout should shoot the big enemy who just shield-rushed the Knight, because the big enemy can't shield block this turn and the Scout has a good chance of killing him. The Knight needs to get off the ground and the Cleric needs to heal him. The Wizard needs to turn around and cast Create Fire in the hallway behind us to hold off whatever is running up from behind. The Swashbuckler needs to engage those two orcs before they get to the Wizard." Each plan just covers a single set of actions, and is mostly guidelines and objectives and less specific actions.
Support Your AlliesWhat to do if none of this works? If you yourself are telling your allies what you intend to do and they don't respond, and you stand still and wait while they go off in separate directions, and no one makes a plan or sticks to it, what can be done to improve the situation?
If nothing else, each player should try to support their allies. If the Barbarian goes charging off into unknown territory, its probably better for everyone else to follow him - or for at least one or two people to follow him - then to let him get killed all alone. If the first three PCs to act each move in a different direction, the fourth and fifth PCs should probably follow one or two of the first three rather than find their own new directions to go. That's true even if they both see a threat in a different direction than any of the directions chosen thus far.