1. Blank Space. No, I'm not talking about the pop song. I'm talking about having a completely empty apartment space to rent out so that anyone who comes to see the rental property can see the full extent of the dimensions available in the space. Some people prefer having an open area, a blank canvas if you will, to see when they first go to see an apartment. Unless you are fully furnishing the area, many people like to be able to picture where they will put their own possessions. Naturally, it would be a good idea to include any appliances that come with the apartment, but otherwise having an empty area may not be the worst idea for you.
2. Fully Furnished. Yes, this goes completely against the previous idea, but another option for you is to have your model apartment completely furnished with a nice touch of home look. Some people lack the vision to see how a place will look when they move everything of theirs in, and if you furnish the model apartment you can show the best layout to have the largest amount of room open. Instead of having a bed in the direct middle of the bedroom, try pushing it against another wall to show how much room there can be left over.
3. Mirrors and Windows. Windows are always a good selling point to apartments. They let in natural light and offer a view of the outside world. Unfortunately, not every apartment is above ground and so windows may be in short supply. If this is the case, try installing a few mirrors in smaller rooms, as they tend to help areas look larger and also help to brighten up rooms by reflecting the light back in.
Having the luxury of a model apartment is a double edged sword. You will have to get a feel for the area and what works the best in regards to setups for you. Once you get that idea, however, you will likely be able to sell those apartments like hot cakes!