Couple things off the top of my head:
1. "It's not your fault!" has got to be in
the copy somewhere, these people are
overweight and it is their fault (most of
the time) - so this builds rapport
and credibility with you... they WANT to
believe you. And tell them why it's not
their fault.
2. I would acknowledge the other products
in the market place as being good, but not
the answer they need... this builds even
more credibility with you before you introduce
your product.
If you go to
Ben Settle and
sign up to get his copywriter's grab bag, you
can read an article where he mentions doing
this -- acknowledging all the other products
in your market to build credibility... but he
goes into more detail and explains this better
than I.
3. Go here to see an AWESOME weight loss ad
written by a top (and top secret

) copywriter:
Breakthrough Discovery - The Secrets To Effortless Weight Loss