Push your tongue against the top of your mouth and place a finger between your eyebrows and apply pressure. Hold it for about 20 seconds and your sinuses will begin to drain.
When you push your tongue against the roof of your mouth, don't aim for a specific point but rather push it flat against the top. When you let go, you should start to feel a little movement in the back of your throat. It's a very simple trick that gets the job done.