There's probably been other threads on the topic. Hopefully this one is better..
Given the other thread about clues for when the PCs are out-gunned, it seems like a thread on ideas for enabling running away would be handy.
I'd like to solve the problem from a system agnostic approach. It's not about Running Away being broken. It's about running encounters where Running Away can be feasible.
My premise is that PCs don't run away because they don't think it will work.
My proposal is that GMs should design encounters so Running Away could be feasible.
Here's the ideas I have thus far:
Make it possible to know the strength of the opposition
Whether you just tell the PCs, give them a clue, or hope they think to investigate first, you need to know what in-game signs there are of the encounter's challenge compared to the party. Big muscles, glowing weapons, signs of high damage output, etc. If you don't, then how would the party ever know if they should avoid this encounter or not.
In new gaming parlance, this is called "railroading". (I'm not saying it's always a bad idea.) :/ It also seems like a waste of a DM's time. It's pretty rare to put work into creating a powerful NPC, and then expect your PCs to never face them.
Supply some chokepoints, barricades and doors
As a GM, you know when an encounter is tougher than the party. Examine the set pieces around the area and add some scenery to enable a fallback point that the party could retreat through and slow the enemy down enough to escape. If the map doesn't make escape feasible, the PCs aren't going to bother trying.
I like this idea. Of course, that stuff should be in every encounter, whether PCs can run away or not.
Use realistic motivations for monsters that don't require pursuit
In the real world, hostile animals do not pursue intruders forever. Once they leave their immediate area, they shake their paw angrily, roar, and go back home. Except for police, most people don't chase after an intruder either. There's too much risk in following the enemy, as you don't know if they had reinforcements hiding around the next corner. It's more probable that NPCs will pursue to a certain boundary, then fall back, regroup and decide if they need to posse up with a proper pursuit effort. Either way, this is the break the PCs are hoping for when they runaway from the initial encounter.
In a real-life military conflict, the kind fought before guns that is, most deaths occurred during the retreat. PCs rarely run unless they've been smacked down, and once the bad guys see bleeding, weakened, spell-depleted PCs run away, they're thinking of plot reasons to kill them, or loot. The PCs need to run away really far before the bad guys give up.
On a similar note, if the losing side doesn't retreat until they've lost a few members, they're weaker and the winners are incentivized to attack. It doesn't help that, unlike mercenaries or soldiers, PCs are much more willing to risk their lives (they wouldn't be adventurers if they wouldn't) and so aren't going to back down because there's a chance they could get hurt as they pursue. The same would apply to at least some opponents.
I'm picturing a group of (fairly weak) goblins who managed to defeat PCs. They had the numbers, or arrows, or (worse) both! There's little incentive for the goblins to not just finish off their opponents unless the PCs have demonstrated some very specific abilities (eg lots of flaming cloaks). And even then... arrows.
There's, of course, a risk of falling into another encounter. This risk is pretty high in a dungeon, but IMO if the battle is making that much noise, perhaps one of the NPCs should have retreated to summon reinforcements. As for the villains, it's pretty rare that the PCs have brought (competent) backup. The risks for following them are low. (Unless the PCs have taken control of traps further back, which is pretty cool, and also sometimes funny.)
Tell your players how the world works
Let them know that not all encounters are level appropriate. Let them know that some encounters are much stronger and that they should be careful, and be prepared to runaway. Also let them know the thinking on why most NPCs will chase to get them out of the immediate territory, but not give endless pursuit.
Maybe we have different ideas of what counts as "immediate" territory, but I think bad guys will usually pursue the PCs for a few minutes.
If the "bad guys" (who might not really be bad guys, of course) are guards protecting a specific point (such as towers with arrow slits), it makes sense not to pursue. However, villains will usually try to kill the PCs. After all, how often do PCs let villains go? If you've won a victory over hard opponents, you're going to want to avoid the possibility of losing to them next time.
I think it's impossible to divorce the system from the running away scenarios, though. Many game systems have poorly-written (or none!) chase rules. If PCs or villains start running away, it's easy to stop them. It can be frustrating to make running away possible.
Here's an example that occurred a few months ago in a Pathfinder game I'm in. Our PCs hid a catapult in the forest near a bandit-occupied fortress. We intended to use the catapult as a distraction when we attacked. We made a good Stealth check to hide it, but it's size penalty gave it a middling result. A few elven bandit scouts barely spotted it (I think they rolled a 17+ natural on their Spot/Perception checks), and while we were a distance away, they attacked.
We arrived as they were trying to destroy our catapult and kill the crews. There were three elves, and in a few rounds there was one, trying to flee. He had great Stealth and a slight speed boost, so the DM wanted him to get away. We were having
none of that.
My druid PC has good Survival and Perception, and could cast Longstrider. He could use an ability to give him low-light vision and Scent (a weaker Wildshape ability, it only lasted 1 minute). His animal companion, a small bear, had speed 40 feet (at least as fast as the elf), low-light vision, and Scent. When you're attempting to use Stealth, you move at half speed or you take penalties. When you track someone, you move at half speed or take penalties. So a slow-moving chase went on. The rules were inadequate for the situation, but all the advantages laid with my PC, and since the rest of the party was following, it didn't matter if the elf could (somehow) ambush my PC and hit him with a few arrows. As soon as he broke Stealth to attack, he was dead. So we just followed him until he realized what would happen. He attacked. He died.