Keeping talking and keep bickering! If something annoys me, I can't hold it in and neither can DH. We both say it as it happens, we sometimes argue about said thing or talk about it, and then we are quite good at just moving on and forgetting about it because we want to enjoy our day together. With bigger arguments, we always make sure both of us say sorry and hug (bit cringe I know).
When you know the other one is struggling, just quietly support them. That's what my DH does for me and I try and do for him. Although it doesn't come as naturally to me admittedly.
I think its different if someone has cheated or been abusive though.
And I've only been together 10 years and married 2! So I've still a lot to learn!!