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Probably the best way to advertise is make a website and start posting help in student forums. Then just post your link in the signature. If you give helpful and knowledgable advice students will click through to your website or blog. Then hopefully will continue on to set up some lessons.

you can post classified ads at several places eslteacherboard.com is probably your best shot. Dave's eslcafe.com student's forum is where I'd start giving help.

There are quite a few websites that offer these services. You should be able to sign up as a teacher and that will give you a good idea about what to do and how to go about it on your own -- while you get paid!