How to unlock additional kart parts

Here's the unlockable character list from GameFAQS.com:

Unlockable  How to Unlock
=========================
Daisy       1st in Mushroom Cup 150cc
Honey Queen 1st in Banana Cup 150cc
Lakitu      1st in Lightning Cup 150cc
Metal Mario 1st in Special Cup 150cc
Mii         Place 1st in all cups in one CC level.
Rosalina    1st in Star Cup 150cc
Shy Guy     1st in Shell Cup 150cc
Wario       1st in Flower Cup 150cc
Wiggler     1st in Leaf Cup 150cc

If your air speed is equal to another speed, it's usually better to stay in the air. This is because you can cut hills or turns that you can't while on land or water. For example, take this crude diagram:

Here, Player A (red) jumps off and glides far, landing farther up on the hill. Player B (blue) jumps off and dives quickly, landing lower on the hill. Player A landed farther up the hill, having a large advantage over Player B.

A good example of a situation of this is the first jump in Rock Rock Mountain. Just after the jump, there's a hill that slowly goes up. It's better to glide as far as possible to land as far up the hill as possible.

Also, this holds true in situations like this:

(That blue... thing is supposed to be a flight ramp.)

Here, the grass is cuttable by flying over it, rather than just hitting the ground.

Even with these things, sometimes taking the land route is still the better option. For example, flying through a walled canyon with turns is pointless, since there no flight advantage, and it's better to get the mini-turbo boosts from ground travel.

To sum it up: generally, the shortest route is the fastest, unless there's a very large difference between land/air/water stats.