Fake It with Paint
I will often paint the walls, ceiling, and trim in the same color. Lighter colors make a space feel larger and airier.
Float the Furniture
Don't push all the furniture against the walls! Float some pieces when creating conversational seating arrangements. Choose pieces that can help to create distinct zones in a small area without taking up too much visual space, like clear acrylic pieces.
Edit, Edit, Edit
It has been said before, but it is so key: Edit, edit, edit. And when you're done editing, edit some more. If your space is full of stuff, it will never feel spacious.
Divide and Conquer
The best way to make a small space feel spacious is to make sure that you have designated areas to make everything seem larger. A studio, for example, should have a bedroom area, dining area, kitchen area, etc. Use different rugs in some of those areas to help make things feel larger.
Try the Rug Trick
I like to use a large rug to cover the majority of the floor. It will draw your eye across the entire space, making it appear more spacious than it actually is.
Stretch the Space
A smaller room calls for full-size rather than smaller furniture, and lower seating heights. Color rather than white walls always makes the space feel bigger, too.
Mirrors can add depth to a room and also can add more light. I always hang mirrors where they can reflect a window.
Fool the Eye
Hang roman shades behind the drapes -- also at ceiling height, but rolled down to just below the top of the window -- in order to trick the eye into believing that the windows go all of the way up to the ceiling.
Be mindful of visual weight when you have a small space. Try a sofa that has exposed legs versus a skirt, or a pedestal table for a dining room instead of something with four bulky legs.
Play with scale. Try using larger pieces versus multiple, smaller pieces in a room. Instead of multiple chairs, try a love seat, and pair it with one larger art piece on the wall above instead of an art collage.