Fight fair: don't say things in the heat of anger that are only meant to hurt the other person. Don't bring up past mistakes or fights; leave the past in the past. Learn to bite your tongue and let it go (I learned this from dh who is much better about it that I).
Talk about money. Find out what your spouse's spending habits are before you get married. Discuss how you intend to handle working and parenthood. Talk about who stays home with the kid, and if both work what you expect to pay for daycare.
Be prepared to either ignore your parents or tell them to butt out, make sure your spouse is ready to do the samething. Don't run to mom or dad when there is a fight and badmouth your spouse.
Make sure you are both in the same place in regard to children. If someone really wants or doesn't want children you probably will not be able to change their mind. Think about what your childcare situation might be. Don't have children to solve marital issues, kids only create more issues.
Learn to forgive. No one is perfect. Remember that when your spouse's quirks are driving you crazy. You have quirks, and your spouse deals with them. Forgive yourself and your spouse when you screw up.
Don't let yourself get into situations where attraction to someone other than your spouse can get you into trouble. You will be attracted to other people, crushes are okay. Make sure they don't go further.
Be realistic. Living with another person is hard and there will be ups and downs. The key is to remember the ups, and work from there.
Don't be in a rush to marry. If the love is real the wedding can wait.
Edited to add: married in 1987, working toward 21 years