Schedule exercise into your day just like any other meeting or work-related outing. That way you'll be more likely not to cancel or double book yourself. Schedule your workouts on your work calendar, or on your iPhone, blackberry, wherever you keep your daily activities and meetings.
Then, treat it like a meeting. If someone wants to meet you for cocktails after work, let them know you've got plans right after work (your workout!) but that you can meet them later, or during lunch. Find ways to keep the appointment with yourself. After all, taking care of ourselves is priority number one!