Depending on the type and size of your event, you'll want to consider hiring first aid and security. In most cases, both are mandatory requirements for getting your municipal permits if you serve alcohol.

Depending on where you are located, there are most likely numerous companies that provide first aid and security services. Do some research and plan on meeting with both to get a feel for what they are like.

Depending on the size of your event, you may want to consider setting up a security/first aid headquarters. Doing so will provide a place for your first aid to set up a temporary triage, and for your security to deal with any issues they may have.

It is often a good idea to let the local police and fire services know the details of the event. Creating these relationships demonstrates to the authorities that you are putting on a professional event and allows them to plan accordingly.