Commit only part of a file in Git

Question

When I make changes to a file in Git, how can I commit only some of the changes?

For example, how could I commit only 15 lines out of 30 lines that have been changed

Answer 1

When I have a lot of changes, and will end up creating a few commits from the changes, then I want to save my starting point temporarily before staging things.

Like this:

$ git stash -u
Saved working directory and index state WIP on master: 47a1413 ...
$ git checkout -p stash
... step through patch hunks
$ git commit -m "message for 1st commit"
$ git checkout -p stash
... step through patch hunks
$ git commit -m "message for 2nd commit"
$ git stash pop

Whymarrh's answer is what I usually do, except sometimes there are lots of changes and I can tell I might make a mistake while staging things, and I want a committed state I can fall back on for a second pass.