Statistics show that 75-80% of the corporate workforce works late 1-3 nights a week. Don't get me wrong here bucko — there are times during the year when you do need to work late — emergencies or a deadline deliverable to a client. Most of the time — working late is due to one of three reasons:

Poor Planning - On your part or your bosses. Remember the phrase: "Poor planning on your part does not constitute an emergency on mine?"
Time Management and/or Procrastination - You spend work time on unimportant activities and push the more important stuff to later in the day.
Showoff - The need to portray to management, the board, your clients, your peers, your staff, or even your family that you are a "hard worker".

Which one are you? Are you a composite of two or even three of these? I was. And it took me YEARS to realize this.

If you can't fit your position into a 40-50 hour window (that's working from 7 AM to 5 PM each day), you need to change a few things. So here goes (in no special order):

You need to SDR - Streamline, Delegate, or Retire - You probably do too much. Leverage your staff and others to pick up the simple stuff. (check out my post)
You go to too many meetings (see this post from last week on meetings).
You haven't set specific boundaries with your boss. You need to train them just like a dog. I'm not kidding - if they try to catch you at 5:30 for an 'important talk' every night, you need to let them know that they can talk to you in the morning.
Stop goofing around at work. No surfing, no personal phone calls, no wandering the office for casual conversation. Do your work! Check this out.
Stop procrastinating. Work on the hard stuff first. Break it up into manageable chunks and get it done. Check this out.
Leave at a reasonable hour. 5 or 6 PM is fine. I know people will notice. But at the end of the day, leaving work to get home for more important activities is critical to your long-term happiness. Work is important — but life springs eternal!