What a simple but great question.
The answer is also short and simple.
That’s really all there is to it. Whatever you did to attract the girl, at a minimum, you have to do at least that much.
You see, the only thing that matters to her is how she feels about you. If she feels enough sexual attraction (as in you keep triggering sexual attraction) and she feels you are a high sexual market value man, then she will long to be with you. Even a walk around the neighborhood is exciting for you.
If, like me in the past, you stop doing the things that attracted her, then her feelings of sexual tension and your perceived value are going to drop.
The less she feels attraction for you, the more she will complain, and nag, bitch and fuss. Sex will go down, and fights will go up.
Understand, every day you should be working to increase your perceived sexual market value.
In order I would:
1) Amp Your Masculine Energy.
This is really your manly, confident core. Always try to be confident, playful, sexually attractive (as in verbally creating sexual tension) and focused on your goal (mission and business). The higher your masculine energy, the more she will be attracted to you. The higher your masculine energy, the more likely she is to feel sexual tension and attraction.
Most of this masculine energy is what you radiate. It is what you believe about yourself in your subconscious. If you believe you are a masculine, confident man she will feel this and that is a primary sexual trigger.
Don’t be needy, insecure, weak, whiny or a wussy.
Weak is never attractive on men.
2) Increase your Physical Fitness Level
You should know that how athletic you are impacts not only how she sees and feels about you, but also how you see and feel about yourself. The more physically fit you are, the higher your sexual market value. I have felt this in my own relationships when I either let myself go, or had an injury that took a long time to heal from.
As a side note, if your relationship is starting to feel stale, starting a new fitness routine where you rapidly increase your sexual attractiveness will not only make you feel more confident, it will naturally make her worried… If she thinks you are looking better then so will other girls. If the relationship has been stale, she will feel the natural desire to try harder or risk losing you.
3) Treat All Women The Same
That is to say, flirt, tease, be playful and talk to everyone. If she constantly sees other girls positively interacting with you it increases your perceived marketplace value. No girl wants to be with the guy that no other girl wants – they all want to be with the Alpha that already has lots of women. Think of yourself as the sexiest and most fun man alive, and then radiate that.
Now, I didn’t say pick up, or cheat with these other women, but keep your flirting and attraction skills honed. The more she believes she can be easily replaced, the less likely she is to ever stray.
4) Never Chase Her
This is what most men do when they feel a relationship is failing. They call more, they text more, and they say more sweet things… all in order for her to say them back so he will feel better. This is weak and needy, and everyone can feel this.
You are better off pulling back. Text less, talk less and let her feel the void and miss you. If you are doing the other steps, she will naturally start pursuing you more. It’s just how women are built.
5) Have Fun
After the courtship and seduction, your role as a man is to be masculine and have fun. Never take her too seriously. In most day-to-day interactions you want to be playful and teasing. Love is supposed to be fun. You simply need to have fun with her, spend time with her, and create an atmosphere for sex to happen naturally. You do these things, and she will fall deeply in love with you and always be chasing you.
Every problem I have had with a girl I wanted to keep has come from not doing these things. Every time I have wanted to get an ex back, revive a failing relationship or seduce a new girl, these are the exact areas I focus on.