Here is a list:
1. Communication
2. Respect
3. Commitment/Staying Faithful
4. LOVE
5. CHEMISTRY
6. Sex
7. Understanding
8. Patience
9. Spending time together as well as giving each other space
10. No BS fighting over crap
11. Spend wisely not foolishly
12. Never buy something expensive without first consulting your partner a Birthday gift or Anniversary would be the only time you can forgo this.
13. Making sure you both want the same things in life for example kids, etc.
14. Both people having jobs even if one of them is part time cause placing financials on one person is a huge burden. Plus it is healthy physically and mentally to work.
15. Compromising
16. Making Sacrifices
Communication, it is all about communication. Almost every relationship suffers because of poor communication. Keep secrets, hiding who you are, etc., it all leads to disaster and destruction in the end. Of course, it it expected for both parties not to agree on everything and possibly argue sometimes, but when you are open and have nothing to hide with your partner, there isn't the stress of keeping anything under wraps and you are more comfortable with one another because you can share anything. My boyfriend and I have no problem even discussing the controversial topics, religion, sexuality, ethnic background, even the topic of communication itself and how we want to establish it in our relationship, it's all easy to talk about. And if you start early on, rather than waiting, it will be for the best. You are talking with a lover, nothing ever has to be awkward and if you show confidence it will definitely make the communication more fluent.
Commitment is a big part, but another benefit of good communication is that, if one side is unhappy, or feels the relationship is not one that will last, the breaking up will be much more mutual and easier to handle and a friendship can even be maintained afterward.