This task can be harder than creating the music itself. Here are a few ideas that may make the job easier. These are not as effective as you may think. If you're looking for a full-time professional, these ads work well. However, for the part time musician, these ads are viewed rarely.
Newspaper ads can get expensive for musicians on a tight budget.
To maximize your investment, check the papers in your area closely to see which ones target the type of musicians you are looking for. Many big cities today have alternative news magazines that cater to a younger audience and may have a large entertainers wanted section. Many such papers will even run these types of ads for free. Word your ads carefully. After you have written your ad, let as many musicians as you can read the ad and get their input.