Look at the major brands and what they use and do the opposite. You need to write your ad as if you are speaking to only one person at a time. That person is your idea candidate. Just pretend that you could say something like this, "Hey Bob Smith, this is the perfect job for you. Read this ad and I know you will agree." Of course you can't do this, but you can keep in mind that you are focusing on one not the many. Secondly, write out at least 20 reasons that this candidate would want the position and try to get at least the top 3 in your ad.
I hope this helps.
Best of luck,
Mike