If possible, try to reproduce the bug (re-do what you were doing before it went foobar) to narrow it down for the devs.
If it's useful, take a screenshot.
Make sure it's not already reported in the forum before starting a new thread.
Try to be clear and concise in your report:
-version of the game the bug was detected in,
-Area affected (GUI, controls, audio, etc),
-Description of what happens, and what you did at the time.
-Description of what *should* happen,
-Is it reproducable? If so, how exactly, step by step.
When a new version comes out, check if the reproducable bugs are fixed (especially if the devs say it's fixed! Heheh).