Get a theme first which, preferably, match your's and your's colleagues (seniors including) or your organization's major expertise or areas of utmost interest at the local or regional levels. Bigger organizations can opt for national importance topics depending upon their outreach, faculty/experts availability and of course funding to cover ALL costs. Constitute committee. subcommittees for various responsibilities. Start at least one+ years in advance and give enough exposure. Cover the invitations through the net and personal connections, ensure a minimum presence. Arrange for delegates stay and transport etc. For feasibility, small organizations can join together. Conferences success is in their perspectives, the region and topic they intend to cover to push the specific subject area/research theme. In my opinion, for a smaller organization, a workshop offers more to learn and train than a conference; however. the choice rests with the organizers. Experts at local, regional and national, as well as international levels, need be contacted well in advance (not necessarily attending!) and their opinions, suggestions and preferably (some) guidance will go a long way!
There are several commercial outlets for arranging the whole conference/event organizer available for fees which can be entrusted with full or partial responsibilities, check the net.