Keep open communication. Let your partner know if you have plans with your friends, and vice versa. If something important comes up for one or the other, be flexible in changing your plans, but also be considerate of everyone.
Plan activities for the whole group. If your partner gets along with your friends, chances are your relationship will flourish as a result. Planning activities for your friends and your partner to all hang out together will help everyone learn to get along and share the time they spend with you.
Plan for a balance. Organize your calendar so that you have some evenings that you spend with friends and some you spend with your partner. Don't forget to plan some time to be alone, too, and dedicate that time to yourself. If you save all your planning for the last minute, you are more likely to run into conflicting schedules with your partner and your friends. Plan ahead to reduce these conflicts.