The easiest way is to use a ramp. Put an object box in your track and set it's type to 'ramp'. Then position it up at the level you want the truck to rise to, and when you drive under it, up you go. It's good to use in combination with ground boxes so you don't fall back down again.
There is another method I've seen other people use where they place a series of flat models on top of each other, but I haven't tried it and don't know all the details. Clawfford has used this but I haven't seen him around for a while.
In all cases, you should put a texture or some sort of marker at the bottom so you know where it is.
[ edit ] got scooped again