Going to take a bit of a different tack here and actually take the subject line seriously!
Qualifications: I used to be a copywriter (someone who writes the scripts of ads for ad agency)
Disclaimer: It was a long time ago and there's a reason I don't do it any more.
There are two types of ads: image and direct response.
Image = raising your profile, making people like you/r product
Direct response = getting people to take action as a result of your ad
Presumably you want to make a good direct response ad. (The best example of "image" ads here would be threADs in Co-Ed, a sadly dying art). That means it needs to:
- have a call to action - when a reader finishes your ad, tell them what to do next, clearly. You are providing a narrative, finish it with the action you want them to do (email, text, etc). The most obvious and successful real world example of a call to action is the "altar call" of fundamentalist Christian sermons/revivals, which end with the preacher imploring the listeners to literally turn their life around as the result of what they say. Your job is a bit less complicated but all the same, you need to see your goal as a call to action and write it accordingly.
- be strong. Use active language, not passive language. Be totally, 100% confident in what you are offering. Do not offer any doubt that you are the person whose services are the best for the reader.
- be very, crystal clear in your language. Do not use "back page style" slang like replacing numbers with letters or using high-ASCII (the funny symbols) letters. Spellcheck and grammarcheck your ad using a word processor. You are a professional who expects someone to give you money for your service. You do not want someone to be unconverted by your call to action because they could not read your ad.
- be very, crystal clear in your intent. If someone responds to your ad positively, what do you want them to do next? Tell them clearly, directly.
- follow up every response, using the means of communication you prefer be used. Do not let *anyone* respond to your ad and not receive a response, as quickly as possible. If you do not respond, your call to action is meaningless because you are saying by your inaction you cannot deliver on it.
- track the tangible results (how many people view each ad, how many people respond to each ad), and note with each entry any differences in that ad - when was it placed? was any text different? were new pictures used? You will see patterns develop. Act on those patterns.
Basically, you need to be an effective marketer. There are MANY, MANY works on the subject. However most marketing texts deal with image advertisements, because ad agencies prefer making them. (Ad agencies prefer them because they usually cost more and have no tangible result that can come back to haunt them.) One of the better writers on direct marketing ads is David Ogilvy. Here's a speech he gives which explains the difference between direct response and image (and why you need to use direct response), but using better words.
http://www.youtube.com/watch?v=fRU1roh932c