I have been married for 12 years now and we are still going strong.

I think these are the most crucial things to keep in mind for a successful marriage:

1. Marry a person who could become your best friend.
2. Marry a person who you are sexually attracted to. Some people say it is the inner beauty that matters but I don't think it is true. Intimacy is part of marriage and at a bare minimum you need to be attracted to your partner.
3. Stay in the same shape (or better shape) as when you married. Don't let yourself go, especially not when your partner keeps staying fit.
4. Talk, talk, talk. The less you talk, the fewer conversations you have, the colder the relationship gets.
5. Have some common friends you can have a drink with or coffee with as couples.
6. Have some friends on your own and let your partner have his/hers. You might have marriage issues time to time and you need friends to discuss these with.
7. Do fun stuff with your spouse that both of you enjoy.
8. Also Do fun stuff your spouse enjoys only.
9. If you have kids, still keep dating your spouse. Dates are crucial.
10. Don’t expect your spouse to do more housework than you unless she/he is staying at home.
11. If you are the breadwinner don't think you are superior.
12. When you argue think about this: will this argument matter in 5–10 years from now?
13. Mutually respect one another’s interests.
14. Don’t lie, cheat and bullshit. Honesty is the building block of a successful relationship.
15. Listen and pay attention to your significant other. Learn as much as you can about him/her. Get to know him/her as deep as you can. This will help you understand him/her.
16. Don’t manipulate him/her.
17. Let each other make choices. One day you choose the movie, next time it is your partner.
18. Go through different life experiences together. Travel together, try new things together. These create strong bonds.
19. Respect your partner and his/her values.
20. Dont try to change your partner too much, it won't work. It is better to choose a person who is way closer to your ideal partner than choose a “hot” one and expect him/ her to act like he/she is not. For example, if you marry a bad boy, you have chosen poorly and soon you will be fighting about him cheating. Bad boys don't change.
21. Before you marry, see if you could live with that person 10–20–30 years from now. A person might be fun and funny, but maybe in 10 years’ time that won't matter to you if she/he will never take out the garbage.
22. Remember, marriage is constant work. If you stop working on it, it will go bankrupt emotionally.

Good luck!

Edit:

Thanks to Magda Selmeci I would like to add a few more items that are very important as well:

1. You need complete trust.
2. Knowing that we were always there for each other.
3. Knowing that our loved one placed more importance on keeping the other happy, than self being happy.
4. Keeping our home a warm, loving, helping and understanding sanctuary.