Happy ad answerer has some sage advice there. I met my now wife through a personal ad that I had posted. She admitted that she'd responded because it made her laugh and struck a few chords with her. I do remember that it was hellish long, providing plenty of information and not a few rants.
If you want to post your own ad, make sure it gives a true picture of yourself. Why not ask a friend to give a frank but pithy appraisal of what he or she thinks is good about you and use that as a basis. The physical attributes aren't really that important - it's the brain that does the most work in establishing a good relationship, not the eyes or the loins. Don't be afraid to have opinions - if you feel strongly about something, you'd hope that a potential partner would at least have some strong views on the subject.
But above all (and this is maybe the hardest thing to get right), try to give a flavour of your wit and humour. And, whatever you do, don't come across as desperate or unlucky in love!
Good luck!