I think it's a good idea to get across what age group and skill level you're expecting. You'll get everything from newbie kids to seasoned veterans that might respond so you don't want to waste their time.
Personally, when I see an ad the first thing that catches me is the title.
"Guitarist Wanted". Then the location. "Toronto" (gotta know where I have to drive to)
An age range "Guitarist wanted, age 18 to 25 for ..." followed by musical styles and what's expected. (ie, I'm not interested in 16 years olds starting their first band so tell me up front what you're looking for)
Below that, any information about a specific image you're looking for.
Then all you need is a large copy of the telephone number on the actual ad with plenty of tear offs below it. Tear a few off ahead of time so it looks like somebody has already expressed an interest and that it's a popular ad.
Then check back regularly and change it up a bit each time so it isn't the same ad sitting there for 2 months. Also check to be sure none of the staff has removed your ad to make room for new ones.
Lastly, get an answering machine or voice mail and don't use a phone number that your mom & dad answer, cause there's nothing more embarrasing than hearing "Joey... there's a man on the phone who wants to play with you?"