One thing I did was think of behaviors that present me as possibly more friendly, smiling, for example. I know when I am driving the car I will feel like I am smiling to someone but, because I am in the car and there's distance, it is hard for someone outside the car to see so I have to exaggerate the smile, make sure my teeth show since they cannot see my "eyes light up" :-)

Training yourself to look someone in the eyes (at least for a moment/glance at them instead of the ground), smile, and say, "hello" as you pass them walking can help a lot. It is action/interaction that triggers the next action so smiling and getting a smile in return, etc. is re-enforcing and can make us appear and feel better. We cannot get people to want to engage with us further if we don't first appear interested in engaging and like we are friendly?