Get the "Safety First" trophy.
Get the "World's Longest Jump" trophy.
Get the "World's Highest Jump" trophy.
Get the "Dominate 125cc Pro Race Season" trophy.
Get the "Dominate 250cc Pro Race Season" trophy.

Go to West Valley Freestyle in one player mode. Go down to the basement and go off one of the jumps. It will take you to a place on top. Follow the way then do the big jump at the end to complete the "Brave The Vertical Drop" challenge. The "Trophy awarded" message will appear and you unlock the Custom Zebra motorcycle.

Go to an exhibition race and set it to "Easy", players to "None" and laps to "One". Race the tracks that you need first place trophies in. This works on race trophies and freestyle trophies.
Contrary to what the CPU will do, you do not need to jump over the bar at all. You just need to reach its height level. To do this, rather than trying to jump over the bar and risk knocking it off, jump using the other two sides of the ramp and just air up next to it. You will know when you have reached the required height when you see your score appear at the top of the screen. All you have to do now is stick the landing.

Go to any tutorial and complete it. Return to the tutorial menu and select any tutorial. Immediately before you begin, press Start and quit. You will get more money. Do this as many times as desired.

Go to the Rock Ridge freestyle track. Go to the left until you notice the chair lift. Head downhill to the lower tower. Look for the stone ramp to the right (while still facing downhill). Get your speed up to about 45 mph, preload, and jump to the tower and stop on top of it. You must now travel up to the higher tower riding on the cables. When you reach the other side, will get the "Lift Tower Challenge" trophy and unlock Elrod the Viking.

Get the "Dramatic Entrance" trophy.

Get the "Impossible Stoppie" trophy.

Get the "Cross Training" trophy. To do this easily, participate in an exhibition race under the hard difficulty setting. Set the laps to "3" and opponents to "0". It is best to use a Super Cross Track because you will need a lot of jumps. Run the race and do tricks until you hit 5,000 points. You will automatically get first place because you are the only one on the track.

Get the "Impossible Wheelie" trophy.

Go to Fox Creek, jump through the fence, across the canyon, and into the ghost town. Pull up next to the church and bunny hop into the graveyard (hold Clutch, Rev Motor, Preload suspension, hold Back, release Clutch and Preload when the suspension is loaded to the max). From the grave yard, jump to the lower roof platform covering the doorway to the first shop. Continue jumping from roof to roof until you get to the end. From there, jump to the back of the building's roofs that you were just on. The last ramp will take you up to a rock land bridge (very difficult to land). Jump from the land bridge down to a ledge leading to an open mine shaft. Follow this passage into the military base and down to Sal's spaceship. Finally run past the ship to a room where Sal is being held in suspended animation to get the "Find Out The Truth" trophy and unlock Sal the Alien as a playable rider.

To get the secret on Fox Creek, get over the fence and follow the white barrels.

Get the "Dominate 250cc Pro Freestyle Season" trophy.
Get the "Dominate 125cc Pro Freestyle Season" trophy.
Get the "Crane's Eye View" trophy.
Get the "Mega Stunt" trophy.