At the main menu, press X, Y, B, A, Back, White, Up, Down, Up, Down.
During an intermission sequence, hold L + R + click Left Analog-stick + click Right Analog-stick.
On Day 55, you will have a chance to only fight drones for 3,000 to 4,000 Neo Yen each. It is possible to make well over 3 million Neo Yen through those fights. Also at the town map screen, place the little "snowboarder" pointer over the garage area. Rotate the Right Analog-stick ten times in any direction to gain 1,000 Neo Yen. Note: You can only do this once per day.
Successfully complete the game to get a "Clear" saved game. A special mech will appear on days that are a prime number. Additionally, the Scobee companies will have a special mech with better weapons available on days that are a multiple of six. For example, Day 6, 12, 18 through 60.
The easiest way to keep your money and not have to repair, is to go back into a gate (such as the one you enter from). If it does not open, bump into it and it will do so.
In the first arena, just run around in a circle, getting any power-ups that manage to pop up. There are side garages you can "pull into" to let your Optical Camouflage recharge. In the second arena, drop into the large circular hole and wait for power-ups, killing enemies that appear. In the third area, start on the gate to the right. Then, hide behind the area where you start.