I have been in quite a few long term relationships in my life (and by long term I mean anything like 2 years. I know it may not seem that long for others, but just take it for what it is.)
The question you are asking is way too general. There are MANY ways to maintain relationships. Observe any couple you personally know. Sometimes, the woman in the relationship controls the relationship by deciding everything. Sometimes, the man is controlling.
I personally believe that a relationship is never 50/50. There is always a chaser and a chasee. One person is giving more, the other less. I know it sounds harsh and cynical, but many times, you can easily observe it in couples you know. A lot of times, when things are great, things are 55% effort given to 45% effort give by the other. Now where does this tie into everything.
Well, the best way to maintain a relationship is simply having and keeping a connection of Comfort. Relationships last so long due to the level of comfort they bring. In the initial stages of a relationship, the act of gaining interest, rapport, and vibes is a must. After all that has passed, there is little game you truly must play. The game is gotten at that point (normally, however, it isn't always the case).
A lot of people stay in crappy relationships simply for the sake of the comfort and ease of being in one. So, my advice to you is, create a sense of comfortableness. This can be done by connecting with that person, creating memories, inside jokes, and intriguing each other on a regular basis. Things tend to old quick, so to keep and maintain a relationship, make it comforting, and make it exciting. Get to know each other and things can easily flow. You get a sense of natural confidence and ease once you get to know each other better and better.