Step 1: Go and buy yourself some colour shapers.
Step 2: Go home and be amazed at how much they improve your sculpting.
Step 3: Send me rewards and money for changing your life! (Or at least a nice comment below).
I'm quite serious about Steps 1 and 2 - Step 3 is optional, but feel free to do it anyway!
These things are amazing, and I can't believe I got by without them before. And before you ask - no, I don't work for the company that makes them!
Essentially they look like a paintbrush, but instead of bristles they sport rubber tips of varying softness, shapes and sizes. The rubber means that your sculpting medium (such as Green Stuff) won't stick to it. At all (well, unless you use a very high ratio of yellow to blue). This allows the nice, smooth, shiny result you see people come up with in their sculpts. They come in soft (which have white tips) and firm (which have grey tips), which I'll mention in a second.
The same company also makes what they call Clay Shapers, which are the same thing but with very firm tips (which are black).
I have a range of shapes, with a couple of very firm shapers, a couple of firm and a few soft. I use the very firm ones to get the putty where I want it on the model and sculpt the basic shape I'm after. Next, the firm ones are used to add finer detail (such as folds in the cloth). Finally, if some details are too sharp, I use the soft shapers to lightly brush over the putty and smooth it out.
Using just these, I can get the basic sort of shapes I'm after. If I need to do finer detail, I do this with my metal tools - I bought Gale Force 9's sculpting kit which, although quite expensive, does come with a spiffy case. My girlfriend tells me it makes me look like a psychopathic dentist, about to torture people:
Anyway, that's how I do my sculpting - you can see some of my examples here (Death's Head veteran), here (Commissar's peak cap) and here (Counts-as Marbo).
I've still got a long way to go before I get really good, but it certainly helps to have the right tools for the job!