What can be done when the headphones won't stay seated in an iPhone? Recently, my iPhone 4's headphones won't stay seated in the jack. I'll push them in firmly, and then a few seconds later (without touching the phone, the headphones, or anything else), they'll pop back out – just enough to no longer make contact, pausing audio playback.

I first I thought it was the headphones themselves, but I've tried an older pair of iPod headphones (without the clicker), and the same thing happens, although not as badly.

If I fiddle enough with either pair, I can eventually get them to stay seated, but I can't tell what it is that makes them stay in on one particular attempt vs. another.

Is there anything I can do? The jack appears to be free of any dust/lint – should I try blowing it with some compressed air? I can't imagine that it's physically damaged on the inside, but I don't know why else the plug would pop out.
@Gerry was right: there was dust or lint in the jack that was preventing the headphones from fully seating. I couldn't see it, but I used the end of a floss pick to clean it out and that fixed it. The headphones stay in so snugly now that I can actually lift the phone by pulling on them (not that I would do that normally; I just wanted to confirm it really worked!).

Amazing that just a little bit of dust could make all the difference. I'm just relieved that the phone itself wasn't damaged somehow since it's out of warranty.