I heard somewhere that if you have strong enough 'why', you can figure out almost any 'how'.  Once you know why you are doing something, you increase your chances of success so much that without a why, there's almost no point.  If we don't know why we do what we do, we are likely to drift and are not nearly as effective. 

So, why do you want to lose weight?  The answer will determine your success.  I think that it needs to be more than 'to look better,' or to 'be healthy'.  That's great and they are worthy goals, but does that inspire you, excite you, give you the strength to avoid that chocolate your co-workers brought to the office?  If it does, great!  But for me, the reasons tend to be more personal:

I want to be around for my children
I want to not have to plan on being the fattest and slowest on the church softball team
I want my clothes to fit.  To know what it is to have a pair of slacks hang like they are supposed to.

Ok, well the last one was a little vain, but the point is, you have to make the goal real.  Tangible goals, measureable results, and focus are the keys to weight loss.  It doesn't matter if it's medifast, taking supplements, or any other program, success comes with certainty of purpose.