I know this seems like a "dummy" topic for the more experienced people, but I work in Auburn, Ca and see a lot of dirt bikes tied down in the back of trucks way wrong. Well, I guess there really isn't a right or a wrong way to do it but I just want to give my input (a.k.a vent)
The way I tie my bike down is with the front tire just to the right of the drivers seat (in the back of the truck, du'h). I then loosely secure the tie downs to the bike (so it won't fall over 
) I then pull the rear of the bike to the right, thus leaving the perfect amount of room to close my tailgate. I place a 4x4 block between my front tire and the front fender so the forks compress about an inch or so (this works a whole lot better than the cheesy plastic ones they sell). I pull the tie downs tight, but not too tight. Also while tightening the tie downs level your bike, I judge off the seat. With the left over strap (of the tie down) I tie a loop knot just bellow the locking system of the tie down (either racheting or pull type) this prevents the tie downs from coming loose. And there you have it, a bike that will not fly out, come loose, or end up on the side of the road while your hauling rear. I know a few people are skeptical about placing a block between the front tire and fender, this works for me and I've never had a problem with it. Even when I hit a 30 mph turn doing 80 got sideways and just about lost control of my beloved truck, a change of underwear was in order after that 
I also believe in closing the tailgate of my truck with two dirt bikes in the back. This is accomplished by having both bikes almost as far to each side (of truck bed) as possible. Then turn the front tires of each bike tworads the middle of the truck and make them touch each other. Then push the rear tire's as far to the outside (of the truck bed) as possible. This is also a good way to keep the handle bars from hitting each other (if done properly) and the tailgate will close with out a problem. Using this system I had two 450r's (with sand tires) in the back of my truck with the tailgate closed. We made record time to Sand Mountain!
Thanks for reading this post, I know I lack certain grammar skills and this post may be hard to decipher but please bear with me. I am always looking for new ideas for keeping that bike in the back of my truck so don't hesitate leave your input and poke holes in my method.
RideHardRideSmart
-V-
P.S- I will include a picture later to clear up my terrible descriptions
