all of the shrooming techniques are explained on www.mariokart64.com/mksc/tips.html

the tree tricking technique can be seen at the japanese video page

the non-tree trick technique goes like this: when you get to the big patch of water, shroom into the water, left of the little islands, and jump into a slide with the shroom, then don't touch the directional pad and let yourself glide. the shroom power will last the entire expanse of the water-glide, the key to this is hitting the bumper at the end of the water which will also increase your shroom length. you have to hit it at full strength though for it to be as fast as possible.

1. shroom into the shallow water, left of the islands
2. use a water glide to extend your shroom
3. hit the bumper at the end of the water and extend your shroom further

or just use tree tricking like in the jap video