Mindfulness is one of the techniques I was taught in an outpatient program I went to for awhile. Do focus on each moment rather than the past or future. Relax, deep breathing, in through the nose, out through the mouth, slowly. Also keep a "half-smile" on your face. Not a silly grin, just enough to keep your face muscles loose, not all tense as when you're angry or worried. The other thing I do is to keep my head up, shoulders back and act like you have important business. Smile at people. Greet them, even. In other words, do the exact opposite of what your brain is telling you to do.

Hope this helps.