If for character-building reasons, story reasons, or game-world introducing reasons you need a party of PCs to flee, the only means I have found successful is presenting them with overwhelmingly superior forces and then rewarding those who fled/surrendered rather than perish foolishly.
Now mind you, this is in an SR game where the players actually have a sense of reality.
It's hard to set up a D&D encounter that promotes IC flight/surrender. Some ideas which might help include:
* Overwhelming numbers: The whole "fight the monster, take its loot" mentality makes generating flight difficult. Try throwing a few moderate/high level monsters against a party, rather than just one big bad evil thing. Undead/monstrous legions help, too. Remember-- Han Solo never ran from a single storm trooper (except in the original original version...), but a squad or two of baddies puts PCs to their heels right quick.
* Overwhelming fire-power: Use cinematic, rather than game-mechanic descriptions of enemy effects. Have the PCs see the dragon's breath lay waste to a village in one cough, or the giant hurtle a house down on an unsuspecting constable. Unleashing that kind of damage on your PCs right off the bat does not prompt them to flee-- it prompts them to die.
* Zone of Safety: Always have somewhere for your PCs to run to. If your players are in the middle of an enchanted forest which spans leagues on all sides and is filled with ravenous T-Rexes your PCs will not run from them. They will die. Provide a cave, a keep, a defensible tree fort, a crag in the rocks-- anything where they can hide, lay low, and be safe. This doesn't have to be a "save point" (e.g., perfectly safe)-- afterall, there's a reason the monsters won't follow the PCs into their "safe place," but providing an out is essential to flight. Without an out, PCs will get suicidal and start rolling up new stats, rather than running away to fight another day.