Everyone who owns a pickup has needed to haul something in it at least once, even if it was helping a friend move -- and you know that they're going to ask if they know you own a pickup! So what are the best ways to protect what you've got in the pickup bed as you're driving down the road?
Secure large items with rubber tarp straps or ratchet straps -- before putting a tarp over them. If these were to shift in transit beneath a tarp cover, they could damage the tarp and allow rain and road dust through.
Cover the load with a truck tarp or another appropriately heavy duty tarp. If you're going highway speeds for extended periods, consider investing in a vinyl tarp.
Use a tarp of appropriate size. If it's too small, it won't adequately protect your load. If it's too large, it will be difficult to secure it properly.
Secure the tarp in place with rubber tarp straps to prevent wind whipping, which will destroy a tarp. You don't want to use anything rigid though, like rope. This can lead to the tarp ripping as wind pulls at the unflexing rope.