Use power-ups only when you have to, for example when you are close to completing a mission. Besides cookies, purchase Cylindria or Spiral as soon as possible to allow Pac-Man to double jump and collect off-screen ghosts. They are very useful in missions where you need to eat many ghosts to extend your time. The spinner versions of these power-ups are a lot more effective. Do not bother with the Magnet ones, as their area of effect is too small.