If you have a baby flying with you:
Take the first row of the zone you are setting in. First rows are usually where the fittings for the baby basinets are available. Also, you will be close to toilets to change diapers and so.
If you are travelling with kids:
Take a side of a row next to the window at the beginning or end of a zone. The windows too keep kids busy and the end or beginning of a zone to be close to toilets because kids are frequent pee-ers and vomiters.
If you want to sleep during the flight:
Take a window seat in the middle of a zone. In the middle to be away from noise (lavatories and galleys) and a window seat so no one bothers you to get in or out.
If you are planning to stay awake during the flight:
Take an aisle seat, will be easier for you to get out and in anytime you want to stretch your legs or just to get little social in the plane.
You are a teacher and traveling with group of kids, like in school stuff:
Group the kids in one part of the plane, and take a seat at the very other side of the plane.
Travelling with the significant other:
Cinema rule apply here, take two seats at the end of the plane either on the left or right. In many wide bodied airplanes the last row on the sides have only two seats due to the shape of the fuselage. No one will be sharing the moment with you!
If you are flying First or business class
This is extremely different from one airlines to another, some airlines even provide shower rooms, bars and other services. You will get the best of the airline in these classes, limited number of passengers and many services. So no worries.
Please note: I said zone, not class. Classes can have multiple zones, especially in economy class in wide bodied airplanes. A zone is usually the area between two sets of doors with a bulkhead separating them.
You can get a good impression by looking at seat-guru they have seat maps for almost all airlines and of each airline. almost all different models they fly with.
You see the doors, the wings and with color coding you can get an impression about the features of an individual plane. Also whether or not your plane as power available for your devices are indicated if available.