The main thing is to talk. So many couples don't share their feelings, and they just build up until they're too big to deal with.
If something's wrong - talk. Tell your partner how you feel, and both of you be open and honest when you feel things aren't right about what you feel needs to change to make things right again.
No shouting, no arguing, sensible talking.
If someone has cheated or is abusive, I think that's different. I couldn't and wouldn't trust again after an affair.