Coins
Get as many coins as possible. The more you get, the more stars you will earn. Besides collecting the scattered coins, get more from combos and stunts that you perform with your driver. Most levels make it impossible to get all the coins in one attempt. Because you have three attempts, Optimize each of them to maximize the coins collected. If you need more coins, farm previously completed stages.
Jumping
Tap the up arrow twice and Johnny will jump off the bike. You can collect a lot of difficult to reach coins by doing this. Do spins and flips then tap the up arrow quickly when at the correct angle to jump and a coin. Land properly after jumps. After a high jump, tap the left button to get the bike into a horizontal position. Tap the right button to get some speed just before you hit the ground. Note: If your falling speed is too fast your bike may break.
Upgrades
Upgrade your initial bike instead of saving for a second bike. When the game suggests an upgrade before the race, improve that particular feature. Tracks with short distances require better acceleration. Longer tracks require better top speed. Tracks with lots of jumps require spin. Stay balanced in your upgrades.