I am in the weight loss niche, and basically you have 2 types of clients, those that want to lose 10-15lbs in a week (usually young women) and those that have been there, done that and realise that they need a lifestyle change. (Usually both men and women who are probably over 40, or certainly over 30).
Each group has to be marketed to very differently, you basically are unlikely to please both groups with one website, so chose which group you will market to first.
I don't think it matters too much which group you choose, but if you are aiming at a sub niche of the weight loss industry your job to write good copy will become easier.
As you are a membership site, look at other diet membership sites out there and try to do what they are doing copy wise.
Obviously the big boys include ediets.com and annecollins.com, you might get some ideas of seeing how they market to their customers.
Personally, if I were you, I would offer your paid course for free in return for their email address of course. Send them their emails everyday through aweber or getresponse (I would do it properly even if you have to pay for a good autoresponder) and then just send affiliate offers to your list instead of trying to sell them your course.
Remember each list member is supposed to be worth around $1 per month to you - if you are offering a good e course, you should be able to market to them easily once the course ha finished, and even before it has finished.
Pay someone on here to make you a good squeeze page and Bob's yer uncle - far easier than rewriting copy again and again...testing, paying for adwords etc.