During the course of the game, you can augment the stats on your Buster Gun with Buster Parts. If you switch Buster Parts just after firing some shots from it (while the shots are still on-screen), you can change the Attack power from the shots. For example, if you fired a volley of shots with a +3 Energy/Rapid stat (thus, six very fast shots), and swap to a Buster configuration with +7 Attack, you will find that, despite the stats changing, the volley of shots is just as numerous and fast as before, but they're actually much stronger now as a result.
Buster Gun Animation Canceling
When you fire your Buster when stationary, then begin moving soon after, this cancels the firing animation (and the internal timer that stops you from firing another shot), but the Buster still fires. You can make use of this by repeatedly tapping Up while standing still and firing at an enemy. Depending on how fast you manage to tap, you can, in effect, emulate a +2 or +3 Rapid stat on your Buster even if you're not equipped for that.
Easy Difficulty
Successfully complete the game under the normal difficulty setting under three hours or with any time under the hard difficulty setting.
Easy money
In the market, there is an orange can. Kick the can behind the counter of the Jet Bakery. A message will appear and say you have received 1,000 Zenny. Leave the room, then return and the can will reappear. Repeat this trick as much as possible to get an unlimited amount of money. Note: Later in the game the amount will be increased to 2,000 Zenny.
Hard Difficulty
Successfully complete the game to unlock a difficulty option at the main menu. Use it to select the hard difficulty setting.