Don't hit below the belt. Don't bring up past history. No name calling. Even endearing terms and names can be hurtful by using a sarcastic tone. Be careful how you use humour.
Laughter is good, but teasing can be misinterpreted.
Listen to one another fully. This includes watching body language. Look at one another while you speak. Don't blame one another or accuse. Try to use 'I' sentences instead of 'you' sentences.