Just beware that placing wheels over the tailgate and blocking the number plate CAN get you fined....On public roads I would be worried, but on fire-trails you should be alright.
One way is to put the bikes front wheel into a corner, tie the handlebars down so the suspension holds it, and then you just need something to hold the back of the bike down.
A better way is to make a rack, such as those seen in a bike store, where you put the back wheel into the rack, tie the front down and Bob's your uncle.
A way to get round the whole number plate covering problem, and secure/protect the bikes more is to create a wooden jig, which fits over the tailgate without covering the number plate, and has small holes cut out the top for down-tubes to sit in. This way you can also permanently put some rubber/foam there.
Another method I have seen is to use a wooden sheet with beams sitting spaced about tyre width. Wheels sit on their, and the wheels are held in place by the wooden beams. All you need is a bit of rope to stop the jumping out.