Often times in a small space, the impulse is to paint everything white, and plaster the walls with mirrors. While light colors and reflective surfaces will certainly help, always remember that everything in moderation will serve it's purpose without seeming like overkill.
Opening a Small Space: Mirrors
Starting with the basics- yes, mirrors help! But, we don't need to go wall to wall here. I like to place mirrors on walls that are adjacent to a window. The natural inclination is to place the mirror opposite the window, but placing the window adjacent to the window lets light bounce through the space, rather than reflect back out. Of course, there are other things to consider- whether there is room for a mirror on an adjacent wall, where activity will take place in the room, and more.
Opening a Small Space: Color
While the standard thought is that painting everything white will brighten and open the space, it can also create such extreme monotony in a space that it's like walking through a snowstorm. It may be bright and white, but it is not spacious. The color that I find creates the strongest feeling of expansiveness is (drumroll please)... blue.
And, while we're on the topic of color... think of it like a voice. A loud voice in a small space may seem deafening (if you don't believe me, come eat dinner with my children in our small dining space!). But, a loud voice in a large room is easier to handle.
Similarly, many voices can seem overwhelming in a small space, where in a larger space we're able to focus on one voice at a time.
Think this way with your colors. In a small room- if you're looking to create spaciousness, keep your voice down (ie. use softer colors, and less of them.
Opening a Small Space: Open Furnishings
Consider the furnishing when making purchases, and focus on pieces that allow transparency, and are raised off the floor. Keep in mind, however, that using several small pieces in a small space can create a clustered feeling; and providing one heavier piece creates a grounding point.