If you go to http://www.rv.net/forum/ there is a whole section of the forum dedicated to truck campers.  There are a couple of "sticky" topics there that have a huge amount of info.

In my opinion, belly bars are the best option.  Simple and strong.  The camper is then secured to the frame of the pickup.  The stake pocket tie-downs are not good for the pickup box.  The box is somewhat flimsy, considering it is sheet metal spot welded together, and the tailgate is not there to help it hold its shape.  There are other options, too, but I have seen them do damage to the box.  I have never seen damage to the box with front and rear belly bars.  If you are only going to be travelling on paved roads and your camper is fairly light, you may get away with the stake pocket tie-downs on the front corners of the box, but I wouldn't use them on the rear.  I want the camper mounted as securely as possible, because the point of having the pickup camper to me is that I can go (almost) anywhere the pickup can go by itself.

To secure the camper to the belly bar, I just use chain and turnbuckles.  They are simple, cheap, and easy enough.  There are faster things on the market (FastGuns, I think they are called), but they are quite spendy.