Define your relationship and be clear about your intentions. How you act in the early stages of a relationship will certainly be different than your actions a year or two later. Although your feelings may change, misleading someone in the early stages of a relationship is an immature way to start out.
Make a commitment to one another and stick to that commitment. Relationships are not all meant to become marriages, but honesty and commitment is the only way to determine if your relationship is ready to end or go on to the next level. If you are constantly looking for an alternative, you can never appreciate and nurture your current relationship.
Give your partner room to breathe. A relationship cannot grow if you never spend any time apart. Part of being in a mature relationship is trust. If you cannot trust someone, you cannot have a healthy, mature relationship.
Forgive and forget. Nothing ruins relationships faster than holding onto the past. If you have chosen to forgive your partner for something and continue your relationship, than you must put it in the past.
Focus on good qualities instead of bad ones. Your partner may forget to call if he's going to be late or leave his socks on the floor, but the roses he brings home or the lawn that he meticulously takes care of is a much better focal point.
Know when to let go. Mature relationships do not necessarily end in marriage. If you are unhappy, be honest and let your partner know that you are ready to move on.