Compromise, respect, apologise after an argument even when you are in the right, compromise, empathy and understanding and of course not forgetting compromise. In a successful relationship you're not going to get your way all the time, there are going to be times when you know you're in the right but so do they know they're right so you find a solution that works for both of you.

Try to think about what's upsetting them and can it be fixed by some action on your behalf, if it can then do it. For example my wife lost a bracelet that held great sentimental value while on holiday. I knew that every time she thought about that holiday the loss of the bracelet will come to mind and eventually spoil the memory of otherwise happy times. I found a very, very similar bracelet on eBay and then had it engraved with a message that referenced the sentimentality that went with the original. Now when she wears the bracelet or thinks of the holiday I'm remembered with fondness because of that action.

Think about what you can do to make them happy and do it, it will make your life happier and easier too in the long run.