There is no way to "enable" running away. The rules already allow you to do so and provide enough ways to help you with it through items and spells (at least in 3E).
And unless the DM specifically plans to make it impossible to run away it is, most of the time, a viable option.
The one thing you need to do is to get the players to comprehend that running away is not equal to loosing the adventure and that they can continue adventuring quite successfully. And if they still don't run away then also tell them that you don't do scripted "The PCs have tu run from this one" encouters but that they have full control over the tactics they use.
Also, get them to talk about running away openly. What keeps many player from running away is that they don't want to leave the others behind.
And if they still don't run, then they simply won't run. Nothing for a DM to do about it.
Thank you for replying on topic. I think you're still not accepting the premise of the problem (that there are problems with the rules of many systems and editions that make retreating unfeasable in situations where it should be), but that may simply be that you personally haven't encountered this problem. Janx and I however, aren't alone in having encountered this situation. It's not always just a simple matter of the DM has made it impossible, or the players refuse to or don't realize they can retreat. There are tactical effects of many systems and editions rules that make retreating a less tactically attractive option than just fighting it out.
However, You've made some good points here. I think the thing of primary importance that you talk about here, and others have mentioned, is clearly talking to your group about retreating, how the rules of the game or edition one is using facilitate it, and that they need to always keep it in mind as a tactical possibility.
Something that might help the rules is houseruling some of the terrain rules. One problem with many systems and editions rules is relative speeds between some monsters and the PC's. Terrain can help, but according to the rules, terrain usually only reduces everybodies speed by the same percent/fraction...which leaves the monsters still faster than the PC's. In reality, certain terrain should be able to level the playing field much more, and in certain cases affect larger creatures much more detrimentally than smaller creatures (like PC's

). So maybe houserules for certain terrain that limit speed to a maximum, instead of just reducing by a fraction, and terrain that provides additionally penalties for larger creatures would help make retreating more viable, and actually work more realistically. For example, medium size pc's should be able to move more easily through dense jungle, than say a large or larger creature like a bear or elephant. Granted though, once a creature exceeds a certain size, it can probably just ignore the effects of most terrain (say something like a Tarrasque).
Another thing that might help in certain situations, is having the group split up during a retreat. If you're combating just one or a couple creatures (though obviously creatures with superior combat abilities than the PC's group), then retreating and splitting up may freeze the pursuers due to not being able to follow everybody. Or, it may allow the greater number of the group to escape, while the creature or creatures are only able to pursue one of two of the group (instead of all). Leaving a rear guard of fighting types that can harry the pursuers and still escape (likely with ranged attacks), can also help the weaker PC's escape to safety first, followed by the fighter types.
