Ok, I've been running my campaign for upwards of 4 months now, and for all the various combat situations I've thrown at the party, I've yet to make them run. From anything. Hell, I've only 'killed' them once(non-lethal that time), even, though I've come within a hair's breath quite a few times.

But the one thing is, they've never wanted to run from an encounter. In all this time, it happened once, and it was nearly a metagame reason why they ran(they assumed correctly that the dungeon guardians needed a password, so they got it instead of trying to kill the things).

So my question is this: How do I design encounters that are threatening enough to chase off a party of level 4-5 adventurers, without making it insta-lethal?

System I'm using is 3.5, but I suspect that most of the advice will be system-inspecific.