Smile more. You don't have to give every person you see a huge grin to be more friendly. However, making a goal to smile at least 30% more on a daily basis, whether you're smiling at people you know, complete strangers, or acquaintances who cross your path, will make you look like a much more approachable, friendly person. Remember when you passed that guy you met that one time and he looked in the other direction and pretended you didn't exist? How did that make you feel? If you want people to feel good about talking to you, then you should smile more at them.
You can also make a goal of smiling more during the course of a conversation.