Wheelie bonuses:
It is possible to get up to 500 second wheelie bonuses with this trick. You will need either need a moped or a dirt bike. With a dirt bike, drive then do a wheelie. When you do the wheelie, you would normally hold the gas and [Keypad 6]. Instead of holding those until you flip over, get a pretty big wheelie then release the gas while still holding [Keypad 6]. If you are traveling at the correct speed, the dirt bike will keep doing a wheelie without the need for your to press the gas. To keep your momentum, tap the brake or the gas. With a moped, you need to listen to the sound. Start to do a wheelie and when you hear scraping sounds on the road, release the gas then quickly press it again before you go down to the ground. Try to keep letting the gas off and pressing the gas until the scrapping sound stops and you should be doing a steady wheelie.
Stoppie bonuses:
It is possible to get up to 150 second stoppie bonuses with this trick. First, get a moped and drive to where there is a long, straight stretch of road. Press [Keypad 9] to lean forward, then use the brake. While you are doing a basic stoppie, release the brake and press the gas (while still holding [Keypad 9]). You should be able to "drive" on your one wheel for a long time. Note: You cannot turn or do anything else.
PCJ 600 and Sanchez work well for wheelies and stoppies. You need to hold the wheelie or stoppie long enough to get a bonus.