Some other things I've picked up:

If the room is small, or just not very large, push everything up against a wall. The floor space makes the room seem a lot bigger than it is. Of course, this means the floor will actually have to be clean.

Try to avoid bright colors in painting the walls, it also makes the room seem smaller.

If the room smells bad despite keeping it relatively clean, and you don't want to use scents, then just putting a fan on works pretty well.