Despite your best efforts to protect the bed in your pickup truck, chances are you're eventually going to find it covered with random leaves, dirt, and other environmental debris. Cleaning a truck bed isn't difficult, but it does take time to get the job done right. Here's a basic procedure for cleaning a truck bed and getting it nice and shiny again.
If you know how to remove the tailgate on your truck, do so first so that you have better access to the edge of the truck bed. If you don't know how to remove the tailgate, pay extra attention to the gap between the gate and the edge of the bed when you wash and rinse the bed.
Sweep Loose Items Out
Get a large broom with relatively flexible bristles and sweep the bed. You want bristles that will move over, around, and through grooves and bumps on the truck bed surface, rather than very stiff bristles that won't bend a bit. Check the gap between the gate and the bed edge for stuck leaves and twigs, and pull those out. If your truck has a removable bed liner, sweep that out, remove the liner, and sweep the main truck bed. Don't replace the liner just yet.
Rinse, Soap, and Rinse
Grab a hose and quickly wash out the bed. Follow this with soap -- mix up some carwashing soap and water, and use a mitt or a soft mop to gently scrub the surface of the truck. Rinse off the soap after that. If you have a removable liner sitting outside the bed, wash and scrub that separately. Dry off the bed and liner. Repeat this for the tailgate as well.
If you're in a drought zone, you might want to take the truck to a self-service car wash where the water will be captured and recycled. You don't have to use the car wash's soap option; just rinse the bed, move the truck out of the bay and take care of soap and gently scrubbing, and then move the truck into an empty bay and rinse the bed out again.
Inspect the corners of the bed and the gap between the gate and bed edge if the gate is still attached. Scrub out leftover dirt with a soft cloth and water from a hose. Dry the corners as best you can, and then let the truck (including the liner and gate) finish drying in the air. Check frequently to see when the truck is done drying out. Once dry, you can wax the truck's painted surface if you want, though you don't have to.
Put Things Back Together
Replace the liner and gate if applicable. Protect the bed in the future with a tonneau cover.
If you want more help cleaning out a truck bed or would like other options for protecting it from future dirt, contact a truck bed manufacturer, like Hillsboro Industries. You'll get the whole range of options available for your truck's make and model.