If you use a land trampoline tying a tow rope to a nearby tree or other object can act as a tow boat and gives the rope some tension. Hold on to the handle and practice your jumps. If nothing is nearby to use as a tie-off point, someone standing on land can hold the rope.
There are, however, risks involved when training on a trampoline. Take it slow and never try to exceed your skill level. Massively high jumps are most likely not needed to pull off a trick. Try a few mid-size jumps and that should be all the height needed.
Never wear a wakeboard when performing trampoline training. Doing so could not only tear your trampoline but also injure yourself if you land incorrectly.
As with many other sports that involve flying through the air, never look down while performing your tricks. Keep your head up and eyes set on the horizon as if you are looking at the tow boat.
For a wakeboard model made specifically for trampoline training see Guru Trampboards.