I am not as anti relationship as most people here. And I've had a couple of enjoyable relationships which I all ended though. Here's what I've always lived by and what seems to work for me. So maybe it's might help someone.

-don't get married. Simple. There is no upside

-cheat. Keep going out if you want to. Keep picking up girls if you want to. Unconsciously you will be more attractive to your partner. It's fun and you reduce any potential pain your partner might inflict upon you by messing around.

-never accept a bad sex life. Women don't contribute that much to a relationship so the sex better be top notch. Otherwise what do you lose by leaving? Now attraction can not be negotiated, but it's your task to make fucking a fucking habit. Tell her gently that you've always been used to a minimum of once a day in a relationship.

-make the important life choices as if you were single. Career, money, travel. Don't limit yourself because there is a girl in your life. This is the golden rule. You will notice they will follow.

-cheat

-dont try to get intellectual stimulation from her.
Haha, what were you even thinking. That's what friends are for. Or even strangers (male). -follow your intuition. Set your boundaries. If she does something you don't appreciate, you tell her. And be willing to walk away. Again... They don't contribute much except for pussy, which is replaceable.

-Always be the one that is better off when you split. Always. This will make sure they don't leave. And if they do.... Well by definition you're better off.

-cheat

-dont take her serious. And let her know you don't. Tell her her opinions are only appreciated on small issues like seasoning the food or buying vegetables, not on life decisions or money matters.