What's the formula for an enduring relation-ship? How come Jenny and I have stuck together for 34 years and not 34 days? What's the glue that keeps it all going? Well, I can think of one or two things.
1) Respect the need for privacy. If the other person prefers to dress/undress in private, or use the bathroom in private, or just be on their own for a while, why not? Total sharing at all times doesn't suit everyone.
2) Accept those idiosyncracies. We all have odd obsessions and habits - hoovering every ten minutes, or scraping out the marmalade jar, or leaving dirty clothes everywhere - and there's no point in trying to change them. That's how we are.
3) Sharing the domestic chores. If one person is doing the lion's share of the chores, and feeling increasingly resentful, it's a recipe for disaster. It has to be even stevens.
4) Maintain the romance. If all the romance has gone out of the relationship, it's dead in the water. There have to be things that keep you a bit starry eyed, a bit soppy, and hopelessly enamoured.
5) Mutual self-expression. It's all a charade if one person is totally doing their thing, while the other is permanently suppressing themself and being what the other wants them to be. You both need to grow.
6) Shared perspectives. Seeing everyday issues in a similar way is important. How clean you expect the house to be, what to spend your money on, how much socialising you like to do. Total incompatibilities can be fatal.
7) Loving each other's bodies. Even as we get older, and wrinklier, and saggier, we still love what we see. We don't hanker after something younger and fresher. Whatever we look like, it's just fine.
8) Communication.* Whatever's going on inside, let the other person know. Too much secrecy and holding-back, too much image-control, will strangle the relationship.
Or to put all that in a nutshell - plenty of give and take. Oh, and plenty of sweet nothings.
* This one thanks to Dave Martin (see comments)