Wii Remote Tilt
Start with your Wii remote tilted strongly forward. The tilt of the Wii remote affects the speed your cow will run, so having it tilted all the way forward (so that the button pad is facing the TV screen) will give your cow a burst of speed that begins right as the race starts.
Look Ahead
Adjust your speed for the shape of the course. Look ahead to see if the track curves; if it does, go a little more slowly so you can make the turns your cow needs without losing too much speed or running off the track.
Hurdles
Avoid hurdles, when possible. You don't get points for jumping over them, but failing to jump or jump high enough will cause you to lose time, so avoid them when possible.
Jumping
Jump high. The higher you lift your Wii remote for your cow's jumps, the higher the cow will jump, and there's never any risk in jumping too high. Lift the remote at least a good foot and a half for each jump for the best advantage and make sure to perform the jumps quickly.
Scarecrows
Hit scarecrows peripherally. You will get points for both stationary and jumping scarecrows if your cow hits them even at the edges of the graphics; you don't have to hit them with perfectly centered, head-on accuracy, and trying to do so will cost you speed.
Speed vs. Accuracy
Adjust your playing depending on whether you're trying to beat an opponent or get a high score playing on your own. With speed play, worry more about getting through the track quickly and avoiding hurdles. With solo play, work on accuracy and try to get as many scarecrows as you can.
Time
Watch the clock. Not only do you need to finish the race within the allotted time in order to avoid failure, but you get a bonus of three points for every second that's still left on the clock when you cross the finish line. For this reason, the best approach to the game is one that combines accuracy with speed.