When my marriage was in trouble, I asked my wife and I, separately, privately, to write what we liked and didn't like about aspects of our marriage: sex, communication, children, money, career, and recreation. For each, we wrote one behavior we would commit to changing and one behavior we wished our partner would change.
Then we showed each other what we wrote, agreed on what each of us would try to change, and that we'd meet in a week to rate our success.
During the week, when we saw our partner doing something good, we'd give a thumbs-up, something bad, a thumbs-down: No lectures, no recrimination. Just the thumbs-up and thumbs-down.
The next week, at a dinner out, we rated ourselves on our success in implementing the new behaviors and asked each other if s/he agreed with our self-ratings. The "meeting" ended with our agreeing on the behaviors that each of us would work on the next week. Key was that each of us proposed behavioral changes for ourselves, not for our partner. The partner's job was only to okay it or to suggest an alternative.
We had those weekly meetings for about two months, and that has been enough to keep our marriage on a far more solid footing. It's been 25 years since Barbara and I had that marriage summit. We've now been together for almost 39 years and hope to continue for a long time.