Understand each other well!
When you understand each other well, you and your partner will fit together. For example: when you are sad, your partner will know the way to make you happy again.
Care for each other!
When you care for your partner, your partner will care for you back. For example: when your partner gets sick, you will take care for him or her. If you are sick, your partner will take care of you.
Be polite to each other
When you respect your partner, he or she respects you back.
No argument or little
If you want to maintain your relationship, you and your partner stop arguing each other. If there are some problems, you and your partner will have a private talk, and find the best solution for each other.
Listen to each other.
If you want your partner to be your friend for long time, you have to listen to your partner. Also, your partner has to listen to your thought too. This makes easier to communicate each other.