Even if you're not close to an AC power outlet, it may still be possible to connect your laptop to an external power source. Adapters that take DC power from the cigarette lighter socket in a car or the in-seat power outlets on some airliners to the voltage used by your computer are also available.
DC adapters work just like the AC adapter supplied with your computer: just plug the output connector into the computer's external power socket, and connect the input plug to a power source. Like an AC power unit, it's essential that you use a DC adapter that provides the right voltage and uses the correct power plug for your particular laptop.
They're far from universal, but a growing number of airlines offer power points for laptop computers and portable entertainment devices on the armrests of some seats, especially in the first-class and business-class sections of their airplanes. The best place to learn which airlines offer this amenity, and which of their seats have power points is the SeatGuru Web site at http://www.seatguru.com/articles/in-seat_laptop_power.php.