Western Cancer wrote an excellent article at ROK a while back with some tips for introvert game. I won't repeat too many of his great points, but I want to underscore the absolute necessity of one of them:
You have to force yourself to show up.
Many people confuse shyness for introversion. I'm not shy -- It's easy for me to talk to people once I'm in the right situation and mindset, but it saps my energy and requires me to recharge alone after. It also distracts me from my own ideas and forces me to use brainpower making useless small talk. Since this is not a pleasurable sensation, my default is to put myself in these situations less often than the average person.
If you are introverted, showing up to the game is 90% of the struggle. Sure you can work on your day rambles, develop ways to show your value, and make yourself a more interesting person, but if you encounter one opportunity to show it off per week you're not going to get very far. Water flows downhill, and without conscious effort you're not going to seek out situations that cause a baseline of mild discomfort.
For example: I hate shopping. I would rather order everything on Amazon.com, go to the farmer's market once a month for frozen meat, and read a good book with the time saved. The idea that I have to get in my car in the year 2013 and spend precious free hours physically picking up items and putting them into a dumb shopping cart is laughable. But I've realized that efficiency sometimes has to take a back seat for social opportunities, and thus I do my shopping like everyone else.
It's an extrovert's world, and we introverts must learn to withstand some discomfort for the sake of approach opportunities and personal growth. Charge it to the game; you must be present to win.