Assuming that you have contacts outside of the corporation where you work, and your name is one that will pop into their heads when they think, "who can I ask to work on this project?", freelance work should start coming to you naturally.
If you don't have those contacts, you need to speak up and get your name out there within the community where you want to find work. Blog about the technology you work with, and show off cool personal projects. Join a local user group. If there is group discussion at the meetings, show that you know your stuff. Go to conferences and make sure you find people to chat with during the day and later on when everyone goes out for dinner and/or drinks at night. In short, it's up to you to show and tell people how great you are before they'll want to work with you.
If you don't have those contacts, you need to speak up and get your name out there within the community where you want to find work. Blog about the technology you work with, and show off cool personal projects. Join a local user group. If there is group discussion at the meetings, show that you know your stuff. Go to conferences and make sure you find people to chat with during the day and later on when everyone goes out for dinner and/or drinks at night. In short, it's up to you to show and tell people how great you are before they'll want to work with you.