Some of the above "advice to others" seem more a reflection on the posters' own marital issues than good general advice. Nevertheless, as someone who's only been married once, and for nearly 40 years, I'll throw some of my own wisdom onto the heap.
Don't be a virgin or feel compelled to marry the first person you have sex with. Satisfying strong physical desires to get horizontal is among the worst reasons to get married.
Definitely live together first. While not a perfect predictor of how you'll get along when cohabiting, it's way ahead of whatever's in second place.
Speak nicely to one another and about one another.
Be kind. This is the person you've chosen to face the world with. So what if you occasionally pick up his/her dirty socks? You've got annoying habits of your own - trust me.
DON'T CHEAT! Know that you and your spouse will find other people attractive from time to time. Run, don't walk, away from these temptations. Infidelity makes underlying marital problems ten times worse. If your marriage is over, end it with grace & dignity without adding damage.
(Fortunately, neither my husband nor I ever cheated, but I sure saw it destroy a lot of marriages over the years.)