Most Helpful Girl
Patience and communication.
Once problems arise people tend to run away from them instead of facing them together. They just assume it's not a good relationship but don't ever think that the relationship may be good but they might not be putting enough effort to understand each other.1
Most Helpful Guy
Here's my opinion on the most important things.
Communication. Talking with each other, and listening to each other is critical. If there's an issue, discuss it (honestly, but respectfully, and with the goal being to resolve the issue not to blame the other for it).
Openness and honesty. Keeping secrets and hiding things from each other can cause a lot of problems.
Respect. If either of you loses respect for the other, it's really hard to recover from that. Be respectful of your partner and always act in a way that makes you worthy of your partner's respect.
Flexibility. No two people are ever exactly alike, so compromise is sometimes necessary. If both people are willing to do that, I think it makes the relationship much better.
Trust. If either of you does something that causes the other to not trust you (e. g. cheating, lying, being secretive), that's also really hard to recover from.
Understand each other's love languages. There's a quiz you can take at www.5lovelanguages.com that shows what types of things make you feel loved. I think all couples should both take that and share the results with each other. Understand your partner's top love languages and make sure you do those things.1