Originally Posted by mrjorts
If you don't want to buy xpadder you can do it with DS3 Tool, which comes with the MotionInJoy drivers you use to get the PS3 controller working. Open the tool, you should have Xinput-Default selected. Click the options next to that. Scroll down to Right stick x+ and change the menu item to Right trigger. Then go to Right stick x- and change it to Left trigger. Use y+ and y- if you want to move the stick up and down instead of left and right.
Edit: this will change the bindings for all games you play, but you can make a different profile before you do it (click new in the panel on the right) and name it "trials", then you can switch between that one and the default when you go to other games.