I realise this is probably covered somewhere in the documentation, but it's something I only figured out a few weeks ago, so here ya go:

When you want to merge an unstable package onto your Gentoo stable system, you often have to set unstable keywords on that package and half a dozen dependencies. Now, I've always done this by adding lines like this to /etc/portage/package.keywords:

dev-python/matplotlib ~amd64

with one line for the package and each dependency package. Now, that makes sense if what you really are trying to say is "I always want the latest possible release of this package", but what I often want to say is "Do what you need to do to get this unstable package installed, and as it stabilises at this or higher version, migrate to the stable versions". Turns out this is pretty easy to do, just make the line in package.keywords look like this:

=dev-python/matplotlib-0.82 ~amd64

so that only the given version will allow for unstable status. The system doesn't prompt you to upgrade every time a new unstable release shows up, so you don't spend forever and a day on upgrades to something that's not crucial to your operations.

There may be something I'm missing, but it seems to work well for me.