What makes a successful marriage? What are common mistakes that should be avoided?

What’s the most important ingredient to have a successful marriage?
What are significant issues that are easy to resolve? What are some common mistakes that spouses make?
What can boys and girls in Shidduchim – or even earlier – do to prepare for a successful marriage?
What should be done when a couple hits turbulence during marriage?
How to handle disagreements over: money, having additional children, conflicting minhagim, where to send kids to school and more