How do you flag code so that you can come back later and work on it?

Todo comment as well.

We've also added a special keyword NOCHECKIN, we've added a commit-hook to our source control system (very easy to do with at least cvs or svn) where it scans all files and refuses to check in the file if it finds the text NOCHECKIN anywhere.

This is very useful if you just want to test something out and be certain that it doesn't accidentaly gets checked in (passed the watchful eyes during the diff of everything thats commited to source control).

Mark them with // TODO, // HACK or other comment tokens that will show up in the task pane in Visual Studio.

See Using the Task List.