A few hints if you want to go for a huge score..
This is not technically cheating, I didn't limit the FPS in this game, it was meant to run at 70fps, some systems have a faster refresh so you could change your video card settings to get a slower refresh to make it a little easier.
Don't waste your bullets when you have a power up, also there are bonuses for accuracy and speed in clearing attack waves.
If you shoot the enemies in certain order you also get bonuses.