Initially, when I got my truck, I took a piece of 2x8 lumber cut to the width of the bed and mounted one of those brackets that have a skewer for the front forks - I was only hauling one bike. The pros were that it didn't cost much, did not require any permanent modification to the truck bed, and could be easily removed so that the truck bed could be used for hauling other things. The cons were that you had to remove the front wheel, then wrestle the bike up and into the bed, where it tried to tip over while you were installing the forks on to the skewer, and I had to remove the tonneau cover I had on the bed to use it at all. Plus I felt that I had to use a cargo strap around the seat post and hooked into the tie-downs on the bed to make the bike secure.
Recently I purchased a bike rack that installed in the 2" receiver for a trailer hitch. It is one of those types that hold two bikes, have the hoops that the tires sit in and a sliding hook that slips down over the top tube.
Personally I think this is the better approach. I can leave the tonneau cover on so that I can, if I choose, throw my other gear (helmet, pump, etc.) in the bed where it is 'reasonably' secure. If I install the bike in the rearmost position on the carrier I can even fully open the tailgate. Again it does not require any permanent modification to the truck, is easy enough to install (one bolt that replaces the hitch pin, which makes it difficult to steal without the correct wrench) and remove, folds up when off the truck so it doesn't take up much room to store, and is easy to mount and dismount the bike. And I got it on sale for not much more than $100 including shipping.