There are two types of power-ups. Some are temporary that are used to get further in a stage (for example, the shield and rocket boost). The second type is a supplementary power-up that is activated before the game begins and lasts indefinitely (for example, to boost ahead or for extra rings from the gold totem). Power-ups cost extra rings to unlock or you can spend in-game currency to buy more red rings. Save your best boost power-ups for moments that allow a good chance for high score. For example, wait until you have increased your Fever level into the patterns with more point bubbles. Also save them for zones that are best suited for them, such as the Mechanical Zone and Green Hill Zone.