OK if you wanna learn a few tips on how to be a master, then just read here...
1. If you hold the item button, your character will hold the item behind his back (only works for red & green shells, and bananas). Great for sheilding red shells or green shells. Also good to do if you wanna get another item.
2. For 50cc, use a lightweight character or mediumweight. For 100cc, use a heavyweight character. For 150cc or Mirror, use a mediumweight character or heavyweight. That's how I beat all the tourneys for each one.
3. Try to learn how to do manual. Practice and stuff and you will ROCK! If you already know how, then you are doing pretty good.
Well, that's all I could think of in about 10 minutes while I'm playing. Hope these tips help. (yes, this is my first forum post)