You keep the machine working in a LTR by doing the same things as you do to attract multiple women and have multiple options to begin with:
1.) Willingness to walk away if she is disrespectful and not submissive
2.) Maintaining a strong frame
3.) Making decision that benefit you first and foremost
Problem with this is that most men cave over time and their frame gets weaker and weaker as the LTR goes on. Woman wants man to put his foot down, he doesnt because he wants to make her "Happy". She loses respect for him by being allowed to have her way, his other "options" are long gone and the relationship diminishes to nothing. She leaves him for a better option, because women are always window shopping, and he then wonders why all of a sudden she lost sexual attraction to him. Meanwhile he lost his frame little by little along the way.
How do you make women happy? By being a man, a leader and having a strong frame.
Wonder why almost every (straight) woman likes being choked in bed? Loves having her hair pulled and her ass smacked? Wonder why they like being dominated in the sack?
Because they want to be led and they want the man to be in control. Saying "no" and doing what is best for you financially, socially, etc keeps her and her emotional ideas at bay. She will pout but she will respect.
Comprehende?
Happy Hunting