Some big ones:
Apologize when you are wrong, and don't wait to do it. You can't believe how important this really is. The act of apologizing is like applying mortar where cracks appear. Don't just assume the other "knows" you are sorry, go say it.
Likewise, admit when you are wrong.
Don't try to "score points", how ever you do it. If anything quickly builds resentment to the other, it is this. If you see a perfect chance for oneupmanship against your spouse, resist.
Say "thank you". Regularly.
Show your appreciation, especially during moments when you aren't feeling it very strongly just then. Even going through the motions on this can get you both through the stretches of time when romance isn't there.
While each person needs to have their private life, this is not the same as an excuse to keep secrets from the other.
Be ready for sex when you're not in the mood, but the other is.
I could go on, but much of it becomes ever more focused towards specific situations.
Regards,
Eric Anondson