Use the gas pedal in a smooth, gradual fashion. Flooring it wastes loads of fuel.
Look far ahead! If you can avoid stopping then do. Give yourself plenty of time and space, if you see the traffic ahead has stopped waiting for a red light or someone to make a turn or something then slow down and they may have made the turn before you have to stop.
Leave the engine running if you only intend to stop a few minutes (if it's safe to) especially if it's a diesel. Starting uses more fuel than idling it for a few minutes.
If you have a load on your truck going up a hill, don't bother giving it more gas in the same gear as you begin to slow down, change down as you here the revs dropping and again be smooth on the throttle.
