Answer:
A lasting relationship consists of trust and communication. You need to be able to tell each other everything, but not pester the other to constantly know everything.
Try to never judge the other person too quickly. Think of things you adore about the person, and keep those in mind when you fight.
Being honest is key. If you feel something about your partner has been bothering you, open up right away. Letting it get to you will only make things worse.
Be supportive of the person, and do special things for them. Remind them of how you feel about them.
This one that may be particularly difficult for most but it can definitely ruin a relationship. Don't be jealous. Of course those feelings of envy may naturally flare up when you see them simply even standing too close to the opposite sex. Keep a cool head, and casually show your affection if this is the case. Other than that, don't let jealousy get the best of you.