First, make the right choice. This begins with doing your best to be someone that another person could live with, and then be absolutely selfish in your choice. You are auditioning people for the highest possible security clearance in your personal universe, and you do NOT want to screw up. It REALLY helps to like each other, and have things to talk about.
Second, commit. Make that COMMIT. Begin by purging ideas like "soul mate" and "one true love" from your consciousness; they are toxic concepts, will inevitably make you wonder if you made the right choice when things get tough. The main thing about the choice you made is that it was YOURS; determine to live with it.
Third, determine to give the world a united front. You are going to disagree; you are probably going to fight, but do it between yourselves, resolve your issues, and stand together against the rest of the world. ALWAYS have each other's backs, no exceptions.
There is SO MUCH MORE. And I am sure that what is above is really probably more than three things, but this will have to do.