Be RICH. This is important for you, but not for her. For her the number 2 rule follows.
Spend MONEY on HER. This is the most important thing for her, whether you are rich, have any money, or must lie, steal, or kill to do it.
Be TALL. Of course you have no real control over this, but if you don't do it, she will secretly and forever resent you for it and it will come out of left field to smite you. Preferably be about 1 foot taller than her - not for comfort or aesthetics, but because you are a trophy and, as always, the bigger the better.
Be a MINDLESS ZOMBIE RULED BY HER. Forget what you've heard about submissive women. They actually rule every relationship with insidious and painful, passive-aggressive, guilt-evoking, whiny, crying mind-control.
Have the EMOTIONAL STABILITY OF A 4-YEAR-OLD. She will be impressed and enraptured by your delight at the sight of Hello Kitty, stuffed animals, puppies, kitties, duckies, bunnies, as well as chocolates, shiny jewelry and other trinkets and knick-knacks. Also, she will understand perfectly well if you pout over the smallest perceived slight or slip and demand to be appeased, assuaged, or made up to. If you behave any other way, she will never understand it.
Dress like a PRETTY-BOY GEEK. This will save her the trouble of replacing your wardrobe and dressing you, herself.
Remove EVERY HAIR ON YOUR BODY EXCEPT THOSE ON YOUR SCALP. The sight of a whisker on your face reminds her that somewhere inside you, something is trying to grow. You must not grow - you must be as unchanging and constant as the firmament..
There are NO MORE RULES to making your girlfriend eternally happy. If any situation arises which you feel the rules have not addressed, you are mistaken - immediately refer to the primary rules #1 and #2 - they are the solution in every such case.