I have a yoga teacher friend who has her own yoga studio. She put a giant mirror on one side of wall of the studio. I think it's a smart idea, as I notice that the room looks twice bigger than it originally is.
I think light is another way of increasing space. Lighting fixuture places properly on walls and around the room as well as light colored accents can open up a room.
Vertical stripes on walls and curtains can also make the room look bigger as well as smaller furnitures. Large mirrors reflect the room making it appear bigger.
I agree that a proper setting of lighting fixtures makes a room look bigger. I never tried the stripe and curtain tricks though. Do you set the curtains right on the wall so that it looks like there is other space in the other side of the curtains?
We made sure when we moved into a new house we didn't put too many things on the wall. We left one wall completely blank and just tastefully hung a few paintings.
The only thing I dislike about mirrors is that they are reflective LOL. If I lived alone in a big house, I wouldn't set any mirror in the house. I hate to get surprised by my own reflection, especially at night.