have fun together
This is key to a happy marriage. I've seen many marriages where the spouses don't seem to enjoy spending time with each other. Almost where folks expect they can get married and continue living the single life. In the end, we have to find how we can have fun with our spouses. Like alabama, my wife and I enjoy a lot of the same things so it's very easy for us to go and do things together and enjoy each other's company.
Also, kudos to whoever mentioned Tripp. He brings the marriage relationship all back to the relationship we have with Christ and that relationship problems are heart problems and heart problems are worship problems. This is very true. Everything else is a band-aid. If you want to get at the root of what causes problems, go to the source, don't try to put a band aid on it.
Now, I will say this, I have read through many marriage books and have facilitated marriage classes among couples based on things like Art of Marriage and Five Love Languages. I believe this all has value, but in the end, getting to the root of our issue is key. It's not as simple as "we didn't go on enough dates" or "he didn't get me enough gifts and that was my love language." That's not what causes a relationship to fail. It's much much deeper than that.