I'm in a similar situation as analyst74. I used to be rather introverted, but in the recent years I started thinking of interaction with people I don't know that well a game of sorts.
Oftentimes, people are far more relaxed and natural with their friends than they are with co-workers or strangers. Subconsciously, we assign these 'roles' for ourselves (you with friends, you online, you on the bus, you at the doctor's office etc) and stick to them. Breaking the boundaries of the roles a bit and being the same across the board is what I found to be quite helpful in being more social and just feeling better about life overall. For example, I'll randomly chat up a stranger waiting for a delayed bus; make eye contact and smile at a person random person on the subway (especially if attractive ;P); or talk to or play with someone's kid while on a train (kids are usually very approachable and love playing on a boring subway ride; your mileage may vary though as North American parents seem to think everyone around them is a potential child rapist).
It might feel strange at first, but the more you behave like a friendly approachable being the more people respond with the same. After all, most people, unless they are rather extroverted, are just like you, and are basically waiting for someone else to initiate contact with them.