Some suggestions to avoid creating unnecessary problems in a close relationship:
Avoid arguing with the other unless it really matters.
Avoid “I told you so.” When it is obvious that the other has made a mistake, he or she knows it. Don’t pile on.
If the other is doing or saying something that truly bothers you, say something, but do not criticize or attack. Instead use a gentle and requesting approach, like “I am not criticizing you, but I would like to request that you do or not do ____, because (give a reason reflects on you, not him or her).:”
Compliment the other generously but honestly.
Don’t sweat the truly small stuff.