Fun
This was fun to play. Unfortunately the game hung sometime a little beyond level 40--there's one enemy sitting there that does nothing, my shots go right through it and it doesn't fire at me. I can bump it around by ramming it with my ship but that's all I can do.
Other than fixing that bug, this is my suggestion:
-Change the controls so that you can decelerate rapidly with the arrow keys when you hold down space, but once you're no longer trying to slow down (that is, when the arrow keys you are pressing no longer oppose your direction of motion) let space have no effect so that you can accelerate normally. So space would be a "quick stop but normal acceleration" mode.
The way I got to level 40+ (no deaths, score = 160970) is by whirling around the outside of the arena. Shoot as fast as you can click and keep spinning around the edge so that the enemy shots usually miss you. The only reason to go to the inside is to pick up powerups once you've cleared most of the enemies. It also helps to charge for a bigger shot--if you charge a little for a big shot, and then immediately click a whole lot really fast, then you'll keep shooting that same big shot rapid-fire until you stop clicking (or run out of energy). That's a very quick way to clear out the enemies.