Visit schools and colleges in your community and ask to see the jobs/wanted bulletin board. Take a printout with your information: name, location, phone number and email address, type of lessons offered, general costs and age limits if applicable. Post your advertisement on as many boards as possible. You might consider printing an ad with tear-away phone numbers so people can quickly get your info if they don't have time to write it down.
Post an ad in your local newspaper. You can generally get inexpensive ad space in the classifieds. Put general information such as your phone number, email and type of lessons offered. If space allows, put in a few descriptive phrases such as "10 years guitar experience."
Visit your local music stores and ask to leave your ads or business cards at the front desk. Ask them to consider referring you as a private tutor to their customers.
Place an ad online in a community forum such as Craigslist.com. This can be a great way to reach people who may not attend a school or college and who don't receive a newspaper.
Advertise yourself on Facebook, Twitter and Myspace. Taking advantage of these popular social networking sites can be an invaluable resource. Start a group called "Guitar Lessons" on Facebook and invite people to join. Update your status, inviting people to spread the word about your availability.
Create an instructional YouTube video and post it online. Send it to prospective students or simply post your contact information for those who are interested in personal lessons.