If you have a starport, you can create more room in your silos for money. Go to your starport and start selecting items to be ordered (specifically ones that are the most expensive). Select as many as possible. Then, when it automatically takes money away, cancel the selections and it will give you your money back, but will not fill your silos again.
Easy victory
Do not attack the enemy, let them come to you. When you start, do not build Weapon Factories. Instead, build Cannons and Missile Turrets. It is a good idea to build Trikes and Quads to act as scout lookouts. Try not to build too fast or you will run out of money when they attack. The most important thing is to not put all of your defenses in one place.
Extra money
Go to the sidebar and press X when selecting "X", "Square", "Triangle", "Circle", "Square", "X" in less than four seconds. Press R3 to get an extra $10,000.
Full map
Go to the sidebar and press X when selecting "Square", "Circle", "X", "Triangle", "Triangle", "Square".