Turn right when the track branches, then left, and then right. After traversing the bridge and passing through the porcupines, take a sharp left to rejoin the main track. The total distance covered on this middle route is a little less than that of the left route, but there are more opportunities to turbo-slide on the left route, so each has its benefits. The middle route is usually the best choice, though (I only use the left route for time trials.)
From Thomas "The Destroyer" Robinson. Using the left-right-left route after crossing the bridge can also be beneficial if playing against computer-controlled opponents, as they may follow you, and hit some of the porcupines (this route seems to have more porcupines than any other.)
On the stretch between where the tracks all come together and the egg, use turbo-slides only with caution. The track is quite narrow here, and it is very easy to fall into the gorges.
Save any mushrooms you get for the home stretch. Once you cross the bridge, turn left, and burn your way across the grass. You do not have to cross the finish line while driving along the road for it to count as a lap (you only have to cross the plane of the finish line), so save time by cutting through the grass. Watch out for the gorge on your left, though.
Confirmed Tricks: How to Jump, Slide, and Skid Your Way to Victory
After the three tracks come together, there is a long stretch followed by a u-turn to the left. While driving along this straightaway, stay on the right side of the track. Once you see the end of the fence(to your right), turn your kart slightly to the right, then begin a rapid succession of short jumps while holding left. Once you get to the left side of the track, you should be facing another portion of track, so make one final jump. If you got your kart turned around enough, not only will you land on another portion of track, but you'll be already turned in the proper direction, so you wont slide or bounce off.
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Variation: this trick can also be performed by replacing the rapid jumps with one big powerslide, but I find this much more difficult to do consistently.