I think the secret is to not overly complicate things by being a know it all you know what. To be friendly, supportive, to both respect boundaries and HAVE boundaries. Don't break trust.
And I think the biggest thing is the ability to practice being able to love someone who isn't your image.
Respect is a HUGE factor in a good relationship. My parents had really good relationships w/ both sets of in-laws and so that's how I thought in-law relationships worked. Give and take, with love and respect. However my mil treated me like an interloper from day one. She is extremely toxic and blames me for, well, everything. A pp advised to be nice to everyone your son brings home and that's great advice. Also I know I don't have to say this on this board - but be a good mother to your sons. Alot of the dil's on the board have such issues w/ their mil's because they (the dil's) are picking up their messes in relationship with the sons. Just as it's no fun to have to deal with the baggage of a so's past relationships, it is hard work creating a healthy marriage and fighting against a detrimental message that a dh got from his mother his entire childhood.