One of the most important things is TRUST. Don't accuse your partner of cheating every time they speak to someone of the opposite sex. Males and females can be friends. Just because they are walking and talking doesn't mean they want to sleep together. Bringing up that issue will lead to arguing and will definately end the relationship. If you keep accusing them of cheating when they aren't, more likely than not they really WILL cheat just so your accusations can be true.
Second, don't be clingy. Let your partner have a night out with the boys or girls. Don't feel that you have to be together 24/7. You'll get sick of each other and the relationship will fizzle fast. It may seem great at first spending so much time together but it will eventually bore the two of you.
Be your own person. Don't like every single thing they like. Relationships that involve people of different interests are the relationships that are stronger. Sure, being with someone who has the same interests as you is good too but being with someone who is a bit opposite and being able to be with them and accept their differences shows that your relationship is important to you and you learn to live with your partners differences to make things work. That sort of relationship takes work and acceptance.