While I, myself, am not married, I have five siblings, and have been fortunate enough to view their married life from the outside, as well as the marriages of my friends. So, let me tell you some things.
1. There is probably a 90%, or greater, chance that your marriage won't be anything close to what your ideal marriage will be.
2. Couples are unhappy when one, or both, parties don't get any time to themselves. In other words, don't be too dependent on your partner. You will only wear them down. Give them at least a full day a week to themselves, and a whole day a week to yourself.
3. You all probably already know this, but kids are a real game changer. If the couple already can't be a good team together, then please, DONT consider children.
4. Religion will get in the way eventually if you two are not of the same belief, or can't legitimately respect each other's beliefs. That said, even if you are of the same religion, there's always the chance that one of you will begin to question your faith, and leave it. This WILL cause problems.
5. Neither party should make a habit of complaining on a regular basis. It only makes the other person unhappy.
6. Don't discourage them from hanging out with friends, even if they are of the opposite gender. If they are of the opposite gender, you should be careful about discouraging them, unless there is a LEGITIMATE concern.
7. 40-50% of marriages are likely to end in divorce in the U.S. Consider that marriage proposal CAREFULLY.
8. It is better for both parties to wait until they are at least in their late 20's before considering marriage. By this time, most people generally know what they want in life, and are mature and competant enough to make bigger life decisions. This isn't always the case, of course, but you are more likely to have a successful marriage.
9. If you ever end up having to go to marriage counciling, you should really be questioning why you're even married to begin with.
10. If you like to crack jokes, but can't take any jabs yourself, you probably shouldn't be married.
11. If you have a chip on your shoulder, get that worked out before marriage.
12. Don't marry someone just because they have a desease, and won't likely live to see their 40's.
13. Don't force the BIG question on the other person. it never ends well.
14. Living together for at least a year before marriage can be quite beneficial.
15. If the other party is abusing you, just leave. Even if you "still love them." Just. F*cking. Leave. NOW!!!
16. Getting revenge does not hep the relationship. You are only making things worse.
17. Be frank. Don't give that stupid answer when you mean the opposite. If they continue with that action, you are 50% at fault because you encouraged them.
18. Even if you are always right, don't say it or rub it in. It only makes you look like an egotistic jackass.
19. Don't be selfish. If you are told that you are selfish, but don't believe it, look in a mirror and think again. If you are still told that you are selfish, but still don't think so, look in a mirror and think harder. If STILL, you are told that you are selfish,and don't think yourself to be, look REALLLY hard in the mirror, and think REALLY hard.
20. Rather than keeping record of your deeds vs their faults, keep record of your faults vs their deeds.
21. Don't make the relationship all about you.
This list can still be much expanded upon, of course, as it really only covers common sense basics that, sadly, not many couples put to mind. At least, nobody in my family ever does.