At first, I thought you had to use the boosts immediately, too. Because when you hit the prize bubble, the "prize" boost is instantly placed in your "hands". But you can save it to use later. Just tap the boost symbol on the lower right (for example, if you get a bomb, tap the circle with the bomb icon.) This turns the boost off, putting a regular bubble back in "your hands." When you're ready to use the prize boost, tap the icon. If you have more than one of the same prize, you'll need to tap it again to use just one. Otherwise you'll just keep shooting your prizes until they're gone...