1.  Fly at off-peak times, when the likelihood of getting a good seat, or at least being able to switch seats, is greater.

2. If you have elite frequent-flier status, you may be able to score a premium coach seat at no charge when boarding.

3. For a better chance of having an empty seat next to you, select one on an aisle near the back of the plane next to an unreserved middle seat.

4. Booking a less desirable seat is better than arriving at the airport without any seat assignment. Without one, you’re more likely to be involuntarily bumped.

5. Take advantage of online check-in; most airlines release exit-row and premium seats that were on hold for frequent fliers exactly 24 hours before the flight.

from Travel + Leisure, February 2010

I’ll add a 6th tip:

6. Book with an experienced travel agent who knows the configurations of various aircraft and can make specific suggestions. Barring that, go to seatguru.com where you can research seatmaps and get specific data about recline, legroom, galleys, exit rows and lavatory locations.