I don't always commit as often as I should, but ever since reading Ryan Tomayko's "The thing about git"[1], I use git add --patch/-p to solve the problem of overlarge or mixed commits. It allows you to snip out pieces of the changed files to add, rather than adding the entire file at once. (Perhaps everyone already knows this, but I used git for months without knowing it. So maybe I'll save someone else.)

[1] http://tomayko.com/writings/the-thing-about-git