Depending on the size of your event, you may want to consider having a contract drawn up to manage expectations between both parties and to ensure the space is reserved for you on the day of your celebration.
Some important deals to work out with the venue in advance:
Building access and walk-throughs
Hours you are permitted in the building, including time for rehearsal, setup and break-down (if applicable)
Point of contact
Provides access when needed
Schedules walk-through with vendors
Emergency or backup contact
Seating and tables
Talk to the venue to find out if insurance is required.
Some venues have vendors for food, audio/visual rentals, etc. that they require or prefer are used within their space.