You may have a sales office on the premises. If they are selling the units, they are probably renting them out. You did not state your location downtown but that would be my first course of action.
I would also ask an agent to list the rental on the MLS, great exposure. They can also place a lockbox on the unit to allow more agents to preview the unit.
Craig's List is great, I have had excellent luck with the service.
The best suggestion that I can make no matter which way you go is to stage it, make it look great, trendy, modern, no clutter. Take fabulous pictures with plenty of light.
Catherine Kolodin, GRI