For me, the most important thing about singing in harmony is listening to each other. Some make the mistake of concentrating so hard on their own parts that they don't hear what's going on as a whole and then slip out of tune.

(I learnt this back when I used to sing in choirs and one choir master mixed up the whole choir so that no-one singing the same part was next to each other. I was singing alto and I was sandwiched between a tenor and a soprano with basses behind and in front! It meant you couldn't rely on the person beside carrying you and you had to listen to the whole piece, not just the part you were singing - a good experience.)

Sometimes the best harmonies evolve with instinct and practicing together rather than trying to set out parts, but you can only do this by listening well.