Shark Model from Fundamentals of Digital Sculpting with Blender. First ever sculpt, I am having trouble with getting the smooth C and S shapes like on gills and tail. It is suppose to be gone with a grab brush but I am finding myself in an endless loop of trying to get the round shape and never getting it. 
Of all the Blender disciplines, Sculpting for me takes the most practice.
Try adjusting the size and strength of the brush, as well as moving the view for a better angle.
These are the 3 main points to continuously think about when sculpting.
Keep practicing and you will eventually get it to where you are happy.
It's also a good idea to take a break for a day or two, with fresh eyes and a clear mind often things come a little easier.
Your shark is coming along nicely though, I see a couple of artifacts, one on the eyebrow and a couple by the back tooth.
Keep it going, keep practicing, and keep taking a break if things aren't quite working out.
Oh, the colors and lighting are beautiful in my opinion!
It also takes a while to get a feel for the Grab Brush; it is a fantastic Brush, but try not to make it too small (at least that was my mistake in the beginning). Also, I (like Kent also mentions) find it easier to use the Mouse, when using the Grab Brush.
Like Adrian said, Sculpting takes a lot of practice, but it is really worth the energy you put into it. And your shark is already, maybe 95% there.
I'll just echo what Adrian and Martin have said, it's all about practicing a lot and it's worth the time investment, sculpting is really cool, specially if you have a illustration background. I'd say it helps a lot to draw in pen and paper, those skills translate to 3D. As you practice sculpting, you start to get the feel of brushes and how the digital clay behaves, that's all there is to it, after that it's all about your eye, drawing, shaping, molding, so practice, practice, practice.