Use all of it
If you’re lucky enough to be living someplace with multiple rooms, make sure you use all of them. A formal living room that doesn’t get lived in is a waste of space. A spare bedroom for guests is great, but see if it can double with some under-the-bed storage or even as an office.
Light, cool colors will make a space look bigger; the walls recede rather than encroach. White, cream, light blue-grey, and light lavender-grey are all good options. If you’re addicted to color, you can always bust out a bright trim, or invest in some crazy throws.
If you’re not addicted to color, try a monochromatic color scheme. It will look clean and fresh, and the visual space you create from lack of competing colors will feel like actual space, freed up. You can still find creative ways to insert color, in small doses.
Light it up
If you can’t see corners of your room, it will feel like they’re not there—and your space will look smaller. Maximize natural light whenever possible, opening curtains, installing bigger windows, trimming the hedge outdoors. Beyond that, up the wattage in your flat (and consider CFLs so you don’t up the electricity bill while you’re at it). Use several sources of light on the edge of the room, rather than one large overhead piece.
Mirrors also help light move around a room, so install them around your apartment. Want infinite space? Hang two across from each other, and proceed to trip out. (Just kidding—this isn’t the classiest thing you could do while decorating, but it might be one of the more fun ones.)
Vertical stripes, painted on, will make your ceilings look higher than they are. Stripes on your floor will make it appear to go on much longer than it actually does. Trick yourself into thinking the whole place has slimmed down and stretched out.
I feel like I write this every other week, but the number one way to make your place better—and in this case, bigger—is to get rid of things taking up unnecessary space. This definitely includes too many coffee table books, vases of dying flowers, and knick-knacks you don’t really care about. But it can also apply to furniture: try a few bigger pieces, rather than a large number of small ones.