One of the strength training/nutrition authors I respect had some very good advice on goal setting, especially when there are unknown factors involved. Muscle gain, fat loss and PE all fall into that category.
The upshot of the advice was to make sure that you set goals that you can absolutely achieve. So setting a goal of “x inches by some date” is a bad goal; you don’t control all of the factors, so it is hard to determine if you are succeeding or not. He would suggest a better goal would be estimate the work that needed to be done and set execution as the goal, e.g. “Perform x hours of exercise per week.” Now you can tell if you are succeeding, and once you are succeeding you can examine the outcome and effectively modify your plan if you are not seeing progress in the direction you would like.
This also goes well with other goal-achieving strategies, such as writing them down and keeping them visible. Using the examples above, seeing the first goal on your bathroom mirror every morning doesn’t immediately spur action the way the second does. The second is immediately actionable (it refers to now), while the first is less urgent (refers to the future).
Well, that got a little long. Sorry. (Hope I am saying that about my dick soon :) )