In an era of increasing pressure to do more with less and make the most out of every budget dollar, HST science operations have steadily been able to give its customers more by increasing observatory efficiency. While original mission goals for observatory efficiency were targeted at less than 35%, HST now consistently achieves weekly schedules which are greater than 50% efficient. Furthermore, special concentration on continuous viewing opportunities and science campaigns (i.e. -- the Hubble Deep Field) has yielded efficiencies exceeding 60%. More than fourteen years of applied operational experience and system analysis by HST ground, flight, instrument, and user support systems personnel have resulted in the success. However, these efficiency levels could be even higher were it not for the variety of constraints and unplanned events which affect how and when the observatory can be used. Certain known spacecraft and instrument constraints impact efficiency with little effect on long range plan stability since they can be accounted for in advance. For this class of concerns, planning scenarios can be developed and analyzed to see what efficiencies might be achieved without these constraints. Unpredictable events such as spacecraft safings and anomalies, targets of opportunity and quick turnaround director's discretionary science reduce the overall stability of an observatory's planned use as well as its efficiency. In this paper we will describe various constraints and unplanned events, show their effects on HST observatory efficiency and stability, and discuss specific efforts of the HST Long Range Planning group to minimize their impact.