After I'm done creating my base mesh, I need to split it into different parts so that ZBrush (and my machine) will be able to handle subdividing it to the level that I want it to be. This is also good for planning out your seams in your model. As you can see, I split the model at the jawline, the elbows and the waist...as these parts worked best for this character. I exported each of these pieces as a seperate OBJ so that I may import them into ZBrush for sculpting. Keep in mind, I only moved these pieces out in the image to show that they were, in fact seperate. When I exported them they are still in their old, correct, place with all of their pivot points set to 0,0,0. When you bring these pieces into ZBrush, their pivots can be set to something else...just clear the pivot before re exporting it or projecting normal maps.
Sculpting is, without question, my favorite part of creating characters. Mainly because the results are so fast but I think it's also the stage where you really start to see your character come to life.
Again, starting with the head I import the OBJ into ZBrush, set it to Edit mode and subdivide it a few times. I switch my "Mouse Avg" setting under "Stroke" to a high level such as 5. Start roughing in the big shapes in the face and work your way up from there. Subdivisions can always be added later if you need a more dense mesh for finer details. Add more subdivision levels by pressing CTRL+D. Move up and down the levels by press D or Shift+D. If your model is looking "blobby" this may be because you are attempting to add big details to a model at a high subdivision level.
Define your model as you see fit. It's tough to say exactly what to do in ZBrush as I think, with time, it's a program that will come naturally to you (once you learn the different interface.) "Work away at it until you are happy with your model" seems like a very broad statement, but it may be true :P
I generally just use standard brushes, adding peaks and subtracting to make valleys. To tighten up areas, I switch to the pinch tool and run that over things like seams or wrinkles...Also, to soften up a model (in case you've made a ridge that is very over pronounced, for example) I use the smooth tool.
When you are happy with your model, clear any pivot points that you have set and save the tool. As you can see this is how far I took the different pieces of my model:
Now, take all of the pieces you've created and move down to the lowest subdivision level. Export these pieces. The reason for this is that if you have done serious mesh changes to your model, it will be shown even in the lowest subdivision level and, once you create the optimized low poly model it will be more accurate to the high resolution model.