Some suggestions:

• Let all faculty members in turn choose speakers to invite. This ensures that everybody gets involved, and that the interests of all faculty members are represented.

• Choose a room / building in which people feel well (this is underestimated sometimes!), and choose a time where people have time to attend and do not need to rush to their lectures once the talk is over.

• After the colloquium talks, always have a joint lunch / coffee / supper (depending on the time of the day) in some pub / restaurant etc.. This fosters discussion among faculty members working on different fields, and with the speakers. Try to choose a different location for this every time.

• As others have already said, tell speakers to give talks which all faculty members can understand -- and not only specialists in the respective field.