Marry a person you truly like who truly likes the real you.
Lust will fade. Love will change. But if you truly like someone and when something fun happens, you think "I can't wait to tell...'...that's what you look for.
Praise the little and big things they do right instead of dwelling on the things they do wrong. People who get praise for good things, do more good things. :)
Be honest without cruelty. Be clear and don't give hints as to what you want or need. No one can read minds.
Don't take them for granted or allow them to take you for granted.
Never stop dating even if that date is only once a month. On that date, do not talk about household chores or kids. Go do something fun together.
Give each other space. You should have friends, they should have friends. Not every friend you have should be shared. A well balanced person trusting person is okay with girls or guys night out.
Talk about finances and kids before you have or share either. Someone who saves for a rainy day may be driven crazy for someone who spends in the moment. Neither is wrong in moderation but it can be hard to find a middle ground.
Talk about kids before you have them. Do you agree on how they are raised, punishments, teachings, splitting of the chores? Kids are a DRASTIC change and can crush a weak marriage. Be on solid ground before you have one.