I think it's knowing boundaries. My wife and I enjoy doing some things together, but we both understand that we don't want to/need to do everything together and we can spend time doing what we love.
Well I am in my second marriage, so I definetely dont have the solution, but I know that financial stability sure does say a lot to making it a lot better.
To marry someone you know you love. And to marry someone that has the same interests as you and likes the same thing as you. Also someone that your not afraid to talk to, about anything.