I think one of the most important aspects of a successful circle beyond good solid facilitation skills is preparation. Every drum circle has a variety of factors and variables to consider. The more information you can gather in advance about these elements, the more you can be prepared to facilitate successfully.
What is the setting? Can participants actually be in a circle?
Will there be adequate seating for the expected turnout? If not, is the surface and instrumentation suitable for some people to be seated on the ground in front of the chairs and some behind standing?
Is sound re-enforcement required for the setting? You can always lead with gestures, but in a large outdoor setting, a microphone and good sound system are helpful.
Are there participants who might assist in the case of a large group, to act as group leaders and/or play djun djuns to help ground the pulse rhythm of the circle?
What is the expected age group of the circle? What is the level of experience they have with drumming and/or drum circles? Adjust what you present in terms of rhythmic challenge to the demographic of the group.
What is the reason for the gathering?
Plan ahead, but be ready for anything. Ultimately planning will help tremendously towards a successful event. There will always be surprises. The more you can be aware of what is happening and deal creatively with surprise elements as they present themselves, the more likely you can turn what appears to be a negative into a positive.