I did the live in sin. I waited 'til 30's to get married. Best advice is to expand on vixenbabes comment:
1) Honesty and openness. any problems here will fuck up your relationship for good. You don't want to spend years walking on eggshells, and sweating everything you say or do. I believe when you are most open and honest, you may have *more* arguments - but not the kind that destroy a relationship
2) Similar values. This is a hard one. Look for people with a similar family background. If you're from a very close family - you're spouse should be too. If you want to raise your kid in the church, make sure they do. If you don't want kids... you get my drift. These three areas in particular are critical I believe.
3) Understand it's work. Releasing some responsibilities, compromising more, spending time doing things you don't want to - getting rid of your Porsche 944 so your 5 year old can go to good schools.
All that said. I'm very happy. I've got a great wife and the perfect child. We went through some tough times and WORKED through it, and life is better than ever.
I hope you are as happy as me.
(PS - flirting is not cheating, as long as you both agree it's not)
