Have a Business Plan
Planning a party involves using resources and staff -- even if it's a one-man operation -- so drawing up a formal plan and budget might get the ball rolling before the bills and hours start piling up. Thinking about the original idea of why you want to host an event and who you had in mind to invite can shape the where. A celebration of success like an anniversary or landing a big new client can take place in a restaurant or hall and even at the business headquarters if it fits the occasion and size, while an informational meeting might require a site with audiovisual capabilities and presentation space.
Networking events can take place just about anywhere, and many use a cocktail hour or coffee and dessert format to bring people together. Loud music would be a "don't" if you want to encourage mingling, while interactive ice-breakers or other activities would be the way to go.
Start brainstorming and reviewing your mailing, vendor and personal lists for who must be invited, such as a loyal client, and who may be a boon for future business, for instance an influential member of the community or local media personality.
Setting a budget will help prioritize the guest list and make decisions about the next stages of planning.