Update June 2019: Achievement glitch issue has been patched. Will unlock for those that already have over 250,000 rep after starting up the game and completing a new session.
Rep is scored in a few ways:
Landing tricks (such as backflips and 360s)
Maintaining high speed (of over 45 to 50 kmph)
Sliding your bike (with the right thumbstick)
Nearly missing objects
Completing mini-boss jumps (only found on some tracks with a lot of stunts)
Fakies (riding your bike backwards)
Big Drops (landing a large jump downward in elavation)
Clean Landing (landing on both wheels flat pointed in the direction of momentum)
As you score more points by pulling off tricks and such, you will build up a multiplier that goes from 1x to 3x. Keep an eye on that meter and keep it at 3x by doing small bunny hops in between jumps or other good opportunities to score rep. Spins are the safest tricks in comparison to front or back flips. If you land your bike sideways or backwards, quickly bunny hop again and try to straighten yourself out.
With practice you can easily score 2,000 to 4,000 rep on tracks in the Highlands. Also be on the lookout for the danger zone tracks and helmet cam tracks as they will award more rep for tricks.
If you happen to unlock the Peaks shortcut, it is very easy to score nearly 50k rep on the bonus level the Moon. Low gravity makes jumps higher and longer air time, and hard to wreck even when landing horizontal at times. Be careful though, there are no check points so if you do wreck at any point, the rep score for the level will reset to 0. So earn about 30-40k or whenever you feel the rep earned has drastically been reduced and cross that finish line. Then repeat the process.