If you could give one piece of advice to any married (or soon-to-be married) couple, what would it be?
What is an absolute MUST for any marriage/relationship?
Daily expressions of love, however your spouse best receives it. I love the concept of the Five Love Languages (it works in all relationships, not just marital) because it addresses that each person gives and receives love differently. We can think we're expressing love to our spouse constantly, but if we're speaking only our language and not theirs, they may not be hearing us. If we don't take the time to let our spouse know at least on a daily basis that we love them it's easy to drift apart. That reminder that you love them is also just as much for them as it is for you--it's a way to refocus your thoughts and actions in a way that supports that love.
What is the #1 no-no in a marriage?
"Secret" friends. Without even going so far as to say infidelity, if you feel the need to hide a relationship from your spouse you should ask yourself why. So you say you're hiding it because your spouse overreacts? Then discuss it with them up front. Respect your spouse's feelings--if they reasonably request that you avoid certain behaviors with friends of the opposite sex then you need to respect that.
What is the biggest lesson you have learned from your marriage?
Wow--I have to pick just one again? I'm going to cheat and say that the biggest lesson I have learned is how to be a better Christian because that includes: learning to apologize better and sooner, learning to make sacrifices, learning forgiveness, learning how to be slow to anger, etc., etc., etc.! I think when I was younger I knew that one of the points of marriage was to be a model of Christ's love for the church, but I didn't really get that until I was married and saw how that all played out. I really get it now and constantly find it amazing how the lessons I learn from marriage really apply to Christianity.