Races - there are shorter paths through each racetrack. You may have made it to this level by just racing, but at Stage 10 you really won't win unless you use both your skill and your shortcuts. If you don't already know the shorter, faster path, then do what I did: take it slow and follow the route most of the AI take. Then keep track of your position both before the fork in the road and after the forks come back together (Example: if you're in 5th when you when on one path but when they come back together you're in 10th you should probably use the other path) to see which one is better. The Races are very difficult until you find the right path, but once you know the shorter routes the Races will be much easier for you.
Another tip on the Races (since I think the Races are hardest among Freestyles and Sprints)... Never crash. I know, easier said than done, but here's my point: The AI seems to reverse rubber-band based on how well you play. You could be in first place by a mile but if you crash the AI will rubberband in front of you... by the same token if you pull off trick combos and race fast and don't crash you will see the AI get much much slower and easier to beat.
Sprints - some Sprints have forks in the road, so again you must find the fast path and consistently use the fast path. Also on Sprints at Stage 10, make sure you have analyzed all the components on your ATV to make sure you are as close to the maximum on HANDLING, ACCELERATION, and I would prioritize Top Speed third. Of course your "Trick" rating is worthless here, so it shouldn't be higher than 12 or 14.
Freestyles - I found these to be the easiest, but if you are having trouble getting a high score I would suggest comboing tricks in the same jump. Instead of holding one trick for the 4 seconds of airtime you might have in a single jump, pull three tricks in succession. You will get a better base score, but more importantly you will add a multiplier for each separate trick, even if it's in the same jump. The multiplier is key to your high freestyle score.