You have to do this with the Roach, none of the other bikes are allowed.
My tactic is to tap the accelerator at the start so you just roll over the initial bump and get maximum acceleration down the slope. You may need to lean forward a little to keep your front wheel down.
Just as the slope levels out lean back very briefly then forward quickly to get some forward momentum for your rider, then bail out just before the bike reaches the end of the ramp.
Once in the air I maintain a death grip on R2 and up+right on the stick, I'm not sure it helps but it does seem to keep you in the air a little longer.
Sometimes it goes horrifically wrong (hey, it's Trials) but I've beaten the platinum distance of 135m a couple of times like this.
It makes more sense to see the button movements in action so I've posted a replay of a platinum run for this on YouTube which shows them:
You can also watch the best riders replays. Simply go to the leaderboards for the event, switch to the "Overall" list and view the replays from the top few players to see what they do.