1. Always keep improving
The female innate nature, even while in a relationship, is to constantly look for the best possible mate. A woman will keep a lookout for an upgrade even while she is with you. The thing about this is that both men and women when they reach a new stage in their life, whether it is getting their first job, marriage, relationship etc, get too comfortable and complacent. It's for this reason we often see guys/girls letting themselves go physically, and in other areas of their life. For guys this is a bad thing, so do not lose your frame and always keep improving in looks, money, social status. Every day, keep thinking about how you're going to better yourself, and don't be content with your life just because you have a GF/Wife now.
2. Find someone with similar goals and monitor whether you are on the same page/course in life now and again.
People change as they get older. A girl may like the fun party guy now and she fell in love with you for that reason, but eventually she is going to set her sights on the white picket fence in the suburbs and wants to know the guy she picked is going to work with her to achieve that goal. A lot of guys end up getting dumped because they don't check the phases of their relationship, which can result in you no longer being compatible.
3. Always pass her shit test, put her in her place, and don't stop being the confident, dominant male
Many men have a soft spot for their SO and end up becoming "whipped," as they say. The strong, confident guy she fell in love with ends up becoming a beta bitch who she has control over. This often happens because men become content, fall in love and lose sense of themselves in the relationship and eventually, the power dynamic shifts from her dominating you rather than you her. This eventually results in her breaking up with you and/or walking all over you.
A woman will also throw jabs at you in the form of shit test even while in relationship or instigate fights to see if you will put her in her place. For men who fail to do it, attraction begins to waver and she ends up cheating on you and/or dumping you. Evidence of this can be seen with many married women at jobs who often complain about how they wish their husband would stand up to them and not let them walk all over them.
4. Demonstrate value
Women can take you for granted when you get into relationships with them. There are many stories where husbands/BF cheated on their SO who was not putting out a lot, and the women responded by not by breaking up with them, but by actually trying harder and giving them the best sex of their life. In other cases the man threatens to divorce his wife who stops being affectionate and she changes her behavior towards him in a last ditch attempt to save the relationship.
Guys need to realize women often use sex as a tool to make you want to commit to them and stay with them, and if she perceives you don't have lots of option she will not feel as compelled to entice you with sex. However, men who have lots of options will always keep her on her toes and worried about the possibility of losing him to another girl, and so girls will put out regularly for these guys. Thus, as men you must always show your value by demonstrating to your girl subtly that other women find you attractive. Flirting with other girls in front of your girl on occasion is actually a good thing.
5. Shut down her comparison of your love life to other people
Women often compare their happiness constantly with those in their social circle. Hence, why many women get baby rabies and try to rush relationships down to marriage around late 20's when she was chill before. The reason being a woman will discuss her love life and personal life with those of her friends and they to with her. If said friend is in a miserable relationship, you are looked at a saving grace and she will love/appreciate you. However, if said friend's relationship is better than her own she will complain/be unhappy with her current relationship and want some change now that she otherwise had no problem with.
Example- Her friends BF's text constantly with her friends and sends emoji all the time but you aren't a big texter and before the girl had no problem with it. Now, all of a sudden after discussing with her friends she wants you to be more emotional and text her often because she now feels neglected.
Women are constantly comparing their happiness with other people around them. As a man this is insanity to keep trying to give into her demands because it's every changing and you'll feel exhausted. Thus, as a man you need to shut that shit down and ignore her demands that she was otherwise fine with before.
6. Do not keep doing the same shit over and over
If you keep doing the same activities things become monotonous and eventually routine and boring. Thus, as a man you always need to be open to trying new things both in and outside the bed room. Such as travelling to a new place you've never been before, or trying a new activity like salsa dancing. One thing women hate is boring and if you go into a routine eventually the fun in the relationship dies out and so to does her passion for you. When this happens all it takes is for someone new to sweep in and steal her from you.
7. Listen to stuff you don't care about but she does from time to time
A lot of guys when in a relationship end up having to listen to their girlfriend talk about some random thing that happen to them throughout the day/week, weddings, gossip, and basically some things we couldn't give a fuck about but we do it anyways because it's important to you. While you may find some of the things she talks about pointless it's important to her and she will appreciate and love you for it.