Frequently times they will drop power ups for you when you ruin a wave of enemies. You require fire power ups as much as possible, however beware-- new enemy ships will frequently fly in from the sides of the screen, towards the top. If you attempt to choose power ups as quickly as you clear a wave and they are still drifting around the top of the screen, there is a likelihood you run right into the new ships. It is much better to just wait on the power as much as bounce their way towards the bottom of the screen.