1. Time Together: You have to see one another face-to-face frequently to make it work, you need to be carving out times for dates for you two as a couple. Even if you have to mark it down on calendars, do it! Call each other often too. Basically if you're a girlfriend or a boyfriend, anything you do together and anywhere you go is a date. (etc).
2. Time Apart: You have to be independent also - you need to spend time away from one another. You have to be capable of being a healthy and happy individual without your boyfriend or girlfriend around all the time. You need to balance the time you have - between your friends, family, time for yourself, your hobbies/interests, school (if you're still getting some kind of education), your job/career, (etc). Have your own life apart form your relationship. (etc).
3. Communication: You have to communicate verbally (talk) with one another. About the serious, important, and major issues, and also about the joking, non-serious, unimportant, little stuff too. Anything you want to basically - how your day went, viewpoints, opinions, thoughts, wants, needs, desires, ambitions, goals, your past, how you want your future to be, hopes, dreams, insecurities, fears, wishes, what's going on in your life, (etc). Basically anything.
4. Trust: You have to trust each other for your relationship to work and not be torn apart by jealousy. You need to trust each other enough to not think that they will cheat or do anything else to hurt you emotionally, physically, or otherwise.
5. Support: You must support each other through thick and thin no matter what. Uplift the other person in good ways. Be there (physically, emotionally, any way you can, etc.) for each other during the happy times and the not so happy times. It will make all the difference!
6. Honesty: You MUST be honest and truthful with each other on everything and about anything and everything that may come up. Lies (even if the other person never finds out the truth) will just weaken your bond / relationship and eventually make you grow apart. Especially if you wait a long time to tell him / her. So the best thing to do is to always be honest with the other person and tell them right away.
7. Self-Respect: Only you control how someone else treats you. Letting someone walk all over you, always doing what they want, and having no boundaries makes you resentful over time and far less attractive.
8. Respect: Respect is a biggy. You need to respect and accept each other's body, feelings, wants, needs, and everything else that comes along with being with the other person. Respect is also no abuse or mistreatment or damage or usage in any way, shape, or form, (physically, emotionally, sexually, verbally, etc).
How to Have an Exclusive, Long Term, Mature, Serious (& Healthy) Relationship by wikiHow