Lemme put it this way: Collect every single gun you can find, then go back and do it again. If you collect every single gun powerup from start to level 2, you should have your first upgrade. I added in places in the tank section to get more gun power, so just abuse the fact that gun powerups come back when you switch modes.
Also, there is a place in stage 3 that has 3 flashing gun powerups(that's 9/32). Even more fun is that if you destroy everything, the items will re-appear because the stack which handles all the sprites will reset or somthing. Four trips ought to give you an upgrade.