So let's say you have just recorded a track.  It may have taken you several tries to get one that didn't have any glaring mistakes and where the recording didn't have what you perceive as major flaws.

Stop, if at all possible.  Stop and wait at least 24 hours before trying to decide if it is good or not.  Your ears get fatigued, you get burnt out on that particular bit of music, and even if you played it right as rain, it is likely to sound wrong on way too many levels.  Focusing intently on a piece of music while you play it through even a few times will make you hyper-critical as a listener. 

You will most often by that point be hearing every little mistake (including many a casual listener wouldn't even notice) and almost none of what you did right.

You'll almost always be able to judge it better the next day.  (More than one day, if it isn't a project on a tight schedule)  So at least give it 24 hrs before you hit the "delete" button.

The same goes for later, when you're doing anything like engineering or mixing down.  Following a 24 hr cool-off rule when possible can save you from throwing away work that actually sounds pretty darn good.