Bus Jump: Enter the city:
This trick is slightly difficult. If you try to bunny hop the walls around the course, you will find that there are invisible walls above them. However, you can take advantage of a glitch to get around them. Go up on the roof (top level) and go near, but not off, the jumps. Slowly go up the jump on the left side. Stop your bike halfway up the jump. Then, if you are positioned correctly, you can bunny hop over the wall. You need to do a 180 in the air to go over it. When you are in the city, you can see behind all the buildings, the water tower, houses, etc.

Bunny Hop:
It is possible to bunny hop your dirtbike. While not moving, hold L (clutch) + A (gas) + Down. Then, hold Preload until the bar is full. Then, release L (clutch) and if done correctly, you should take off into the air. Level yourself out and complete the bunnyhop. It may be a little tricky at first, but it is fun to jump on buses and and other things. You can also bunny hop, going slowly (or any time you can pull a wheelie). However, it is easiest when going very slow or staying still.

Additional bikes:
To unlock the option to choose between 125cc and 250cc, go through the game in career mode until the last level. Once there, acquire first place to unlock the option the change your cc's on your dirtbikes in any mode.

Bus Jump: Easy way onto roof:
Ride your bike into the "U" shaped part between the buildings (where the two glass tunnels are above). Ride you bike to the end of the tunnel on the right side. Wipe out or bail while in the right corner square. If done correctly you will restart on the roof.

Las Vegas: Go under the fence:
When you start in Las Vegas, turn around. You should see a light patch of dirt. Go as fast as you can and power slide under it. After you get under it, you can go into the desert or look at the skyscrapers.

Vulture Gulch Canyon: Wallride a cliff:
Select freestyle mode. Starting from the trucks and cars, go to the back side of the arch in the level. Back up slightly and get a good running start. You should be able to climb up the side of the mountain. There will be a cliff up there that you can ride on.

Warehouse level: Secret area:
In the Warehouse in freestyle mode, go on top of the biggest jump. You should see two holes in the wall. They lead to other rooms and levels. To get to one of those levels, go over a large green crate is located (not the area with a lot of green crates, just the one). Go off the left edge of the jump into the corner with a large metal sheet. Bail or just fall when you hit the metal sheet, and it will reset you in a room with a huge quarterpipe, a big tabletop, and two holes in the wall. Note: This does not work every time. Hit the very big jump at a angle, as if your coming from all the big green containers. If hit correctly, you will fly onto the platform and reach the secret area.