Okay: everyone's different so my tips aren't universally good advice. Slip them on over your personality and see if they fit.
1. If you're desperately lonely, as I was when I was younger, here's what you don't know and may refuse to believe: being in a bad relationship is worse than being horribly lonely. So my number one tip is, don't settle! If warning bells are ringing, don't throw a sheet over them, push them into a corner, and try to ignore them. Go on being lonely a bit longer. Trust me, it's better than the alternative.
(Don't settle means don't marry someone who, in some important way, makes you unhappy now -- hoping or trusting that will change. Assume the problem that exists now will always exist. I'm not talking about petty annoyances. It's okay to settle for someone who leaves the toilet seat up, unless that fills you with unbridled rage.)
2. Before you marry someone, make sure he or she is your friend. Don't get married first and hope friendship will come later.
3. Finances, children, family, friends, drugs/alcohol, gambling (and other addictions), politics, and religion can be contentious issues. Have you and your intended talked about them? Are you comfortable with a joint bank account? Separate bank accounts? Do you both want kids? Do you both want to stay childless? Is your mom important to you but loathed by your intended? What if your mom gets sick and has to move in with you? Does your intended hate your best friend? Etc. If you have major conflicts in these areas, don't just expect them to get better.
4. Is there something sexual you're afraid of talking about? Do you have secret S&M fantasies? Talk about it now. If you don't, it will either come out way later in your marriage and your partner will be blindsided by it or you'll live in the closet, which will suck for you and for your spouse. He or she may never know, but your tension will mount, and that will be obvious. Whatever you do, don't say, "I'll just keep it to myself for the rest of my life." You won't -- or you will and you'll be a mess.
5. Make sure you spend time living together before you agree to marry. Some people work as housemates; others don't -- even if they love each other.
6. Get to know each other sexually before marriage. Are you compatible?
7. Picture this: your intended gets into a terrible accident and becomes paralyzed. He or she can no longer have sex with you, and you'll have to take care of him or her for the rest of your life. Are you up for that?
8. Spend a year or two communicating about everything. Everything. Set up a pattern of doing this. There should never be a time, later, when you "grow apart" or one of you says, "He's changed" or "I don't know her any more." If you communicate every day, for the rest of your lives, that can't possibly happen. Neither of you is ever allowed to blindside the other, no matter how hard something is to talk about.
9. Pledge to each other that you'll get help if you need it. Don't be those stupid people who could have saved their marriage with the help of a therapist but didn't.
10. Work on a huge, multi-month long project together -- something really difficult for both of you that required teamwork. Can you be good partners under pressure?
11. Make sure you fight well. You will get into fights. Can you recover from them? Do you fight dirty? Does your intended fight dirty? Is either of you passive-agressive? Does either of you sulk for days afterwards.
12. How compatible are you in terms of clinginess? Do you both require the same level of affection? Or is one of you more independent than the other? How will you cope with that?
13. If there's something that annoys you about your intended, do not go into a marriage thinking he or she will change. Assume no change will ever happen and this is the way it will be for the rest of your life.
14. Are both of you committed to this for life? You never know what will happen, but are you committed, or is this a "let's try it and see what happens" proposition? That might be okay, but only if you both feel that way. Don't hope to win someone over after marriage. What are your expectations about marriage? Did your parents stay together? Did your intended's stay together? Do both of you strongly believe in marriage?