There are a lot of places that dust gets into the bed and it is very noticable when a bed cover is installed.
First, there is a gap in the bed wall at the front (opposite the tailgate)where the section behind the cab meets the floor. The kit I am referring to provided a gasket that seals this gap from top near the bed rail to bottom, across the width and up the other side to the bed rail.
Next, there are foam blocks that plug the gaps on both sides where the stake rails meet the bed under the bed rails along the front and back and above the wheel well humps in the middle. If you take a flashlight and look under the rails, you can see these gaps easily.
Moving on to the tailgate, the kit provided gasket material to run from under the bed rail on one side down along the bed between the tailgate and bed, ending at the bed rail on the other side. They also include other gasket material to seal the ends of the tailgate near the top. This gasket also seals out water intrusion from around the tailgate.
Finally, they include clear stickers to put on the round holes on the bed sides, if your bed doesn't already have covers on them. Mine had covers on all other holes except the drain holes at the front of the bed which I did not cover.
I have no pics because I installed a Bedrug after sealing the bed, but here is a link to the kit: http://www.amazon.com/gp/product/B001B6IF6S/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1
I hope this helps.