You can get multipliers in several ways.
- Big Air (+1)
- Flat Spin (+1 seems like angle must be > 180 degrees and the multiplier increases with higher angles)
- Super Jumps (+1)
- Barrel Rolls (+2)
- Billboards (+1)
- Boost chaining does not give you mulipliers but points. If you do a 3x boost chain you will get: 1000 + 2000 + 3000 = 6000 points!