Respect your friend's point of view. Never betray a trust. Trust, when violated, can ruin even the best friendship. Never resent a friend's success. Instead, strive to admire her achievements.
Flexibility is always important for maintaining friendships.
Be understanding when your friend has a last minute change in plans. When there is conflict, it must be resolved for friendship to continue. Confrontation is one way to resolve conflict. Talking it out is another. A cooling-off period is sometimes necessary when conflict has occurred.