typically, you put the front tire up against the front of the bed of the truck or up against the toolbox. make sure the front tire is perpendicular to the front surface. use a tiedown on each side of the bike, either hook on handlebars or some are strap type (soft tie) that go around the fork leg between the triple clamps or around the handlbar ...
attach the other end of the tiedown to the front and outside of the bed. I installed heavy eyebolts the the bed rail caps to attach to. then you slightly press downward on the handlebars to collapse the front forks a little bit while applying pressure to the tiedown on each side... this ensures the bike is pulled forward and to the extreme outside that will fully stabilize the bike.
some people opt to put a bike shoe in the bed to hold the front tire from turning from side to side... but if tire is completely perpendicular, will hold very firm with the forward & outward tension as stated above.
personally, I don't care for the ratcheting type tie downs and go for the tension / cam type ancra brand.
http://www.ancratiedowns.com/Motorcycle_ATV__Side_X_Side.html
guess it all depends on what size bike you are wanting to haul... have hauled full size street bikes using two tiedowns as described above pulling forward and would also put two on the rear stabilizing size to side... but for a dirt bike, merely two tiedowns to the handlebars/fork legs works fine...