I've had a mortgage with my credit union for about ten years now, and they still have it. And no, it wasn't just resold out from under me without my noticing it; I actually asked one of their reps once and they outright told me that they don't resell their mortgages.

The downside is that as far as I know, I don't have a promise from them that they won't sell it, just a track record of not selling it. Still, if you're worried enough it may be worth considering. I recognize this is not a free option; you'd have to consider it as a form of insurance and consider whether you're willing to pay for it.