1) Play defensively, not offensively.
getting rid of your items on 150cc or Mirror willy-nilly wont cut it. try to keep your shell or a mushroom incase trouble arrives.
2) try to be first on all laps.
on most occasions, a ★★★ rating will determine based on how much laps that you stayed first on. heres the full list of how much laps/track you can miss first on:
50cc: 2 laps
100cc: 1 lap
150cc: 1 lap
Mirror: 0 laps
-OR-
2) time your driving.
time and speed is important. if your slow, most likely you wont get a ★★★ rating.
3) find shortcuts and offline exploits.
Find the best exploits all around. CPU will carry more dangerous items if you do a exploit, but ignore it. It may not get to you (depends on what lap your on.)
4) Get all 40pts.
Points (pts.) is important in Mario Kart. If you want 40pts., try finding shortcuts and exploits step.
5) Dodge more spiny shells.
The most logical reason yet, try to let a CPU ahead of you when someone has a spiny shell, or just simply fall off the course