Clever parties...
1) Concentrate their attacks.
If someone is pounding on something and it doesn't go down, try to pound on it yourself. In D&D, half-dead is nothing like dead. Having two things that are half dead doesn't help you much, and may even hurt you.
2) Defend their weakest links.
The only reason not to be concentrating your fire is to plug a gap in the line of defense so that hard hitters like your archer or spell-casters can be concentrating their fire.
3) Go after the biggest threats first.
Find a way not to get bogged down in that line of monsters with high ACs or lots of hitpoints, and start pegging that spellcaster on the back roll - ready shots, get a rogue up adjacent to them, etc.
4) Work as a team.
A party is cautious together, but never cautious as individuals. A smart party makes sure that the damage gets spread around, takes risks to help other players out. High ac/high hit point characters literally shield low ac/low hit point characters, providing cover to weak characters or characters that have dropped. High ac characters deliberately draw AoO to allow characters in trouble to back out of danger or to put themselves in flanking positions. Healers take risks to deliver healing where it is needed.
The 'aids other' suggestion is a good one. A good party should be using 'aids other' alot, especially out of combat. There are almost always things you can RP doing to make the 'aids other' check reasonable for any challenge.
5) Have a getaway plan, and lives to fight another day.
At low levels this means you back up and drop some flaming oil in a corridor and then run. At high levels this might mean you drop a wall of force, and then use magical transportation or teleport. Whatever it means, a clever group of PCs always knows where the exits are, which they plan on using, and how they plan to get away from the monster. They don't get in trouble and then start thinking about the fact that the monster is faster than the slowest member of the party, especially if someone is carrying an unconscious/dead body. Similarly, the clever party always prepares some fallback position which they can hopefully defend long enough for a long rest.
6) Runs away before they get in trouble, not after its too late.
With half the party lying on the floor and the rest running out of hitpoints is not the time to decide that this isn't a good day to die. If you pressed on when maybe you shouldn't have and, "Oooops. This looks like it might be the BBEG.", don't go down in a pointless climatic battle, start a fighting withdraw immediately. At the very least, you may draw the fight back into a narrow corridor that you can defend more easily than the big open space filled with minions and traps that the BBEG is prepared to fight you in.
7) Plans for trouble.
Too often I see generally clever players only worrying about their one big trick that they use all the time. You need to specialize. But if you create too specialized of a character, what you'll typically find happens is that you overwhelm 90% of challenges, and then in the 10% where things happen that you didn't expect (but maybe should have), you are helpless. Players really need to spend some time thinking about common hazardous situations that they may find themselves in, and what they plan on doing about it. This means things like, "What happens if my character falls into deep water?", "What happens if I fall into a pit?", "What happens if my character is poisoned?", "What happens if my character is grappled?", and so forth. A clever player knows that giving up some basic defenses to move your average damage from 18 to 20, probably isn't worth it. A clever player is all the time asking, "What could kill my character?" Chances are, it won't be a straight forward slugfest that does it unless your DM is the sort that only provides straight forward slugfests.
