Ive got a tendency to modify many things as I go and break them into commits later. Generally I do git add -p until things are too interwoven. In that case, I've ended up with the following strategy:
git commit -m "WIP" &
git revert HEAD &
git revert HEAD &
git reset HEAD^
Now I've got a copy of the changes as a commit and I'm my work tree. I widdle it down until its one change only. Then I do:
git commit -m "WIP" & git revert HEAD & git revert HEAD & git reset HEAD^
Now I've got a copy of the changes as a commit and I'm my work tree. I widdle it down until its one change only. Then I do:
git commit -m "Bla" & git rebase -i HEAD~3 -X theirs
Them order Bla above WIP. I keep doing this "double revert" trick until the WIP no longer applies. Then done.