No, I never took any serious art schooling because my subject matter was always junk according to the teachers. I was always doing creatures, dinosaurs, warriors, ghosts and other fantasy things they wouldn't show in the student gallery. I've never thought of it as art and never thought about any schooling, just messing with stuff until I figure it out. Nowadays there is a school for toy design but it doesn't teach toy sculpting, only basically the "business" end of toys and toy marketing. There's still no action figure/collectible statue sculpting school.
If you're already doing clay cats and crafts that's a great start! Do you use Polymer Clay (super sculpey)? That's a standard for action figure sculpting (now being replaced by wax by most studios) and the art doll scene. It can be obtained at just about any craft store. Make sure it's the Super Sculpey product sold by 1 lb block in the green box (it's all pink). Mix it with a color cube of Sculpey Premo which will make it easy to see the details in your clay and make the baked super sculpey project less brittle.
It will help you to get some good anatomy books which show you the surface (or superficial) muscles. This book is tops in my anatomy stack, you can read the description/order at amazon WAY cheaper than the hardback version from the bookstore!
Visualizing Muscles by John Cody
I have a ton of anatomy and reference books on specific subjects like wrinkles in fabrics, hands, heads, wax, animal anatomy, etc. There's a great number of them which I've picked up just at Barnes and Noble and Borders. Books on drawing work as well for sculpting. Christopher Hart's series on drawing Extreme Anatomy /Cutting Edge Comics is a very good for doing heroic men and women in comic style.
Like everything it just takes practice along with using your reference books while you're sculpting. With polymer clay the project won't go bad if it isn't finished in one day (I've had sculpey projects on my shelf since last summer and they're still sculptable) so all you need to do is go back the next day and work it some more. You'd be surprised how your work progresses if you can plug away day after day until you feel the project is "done". Take a digital photo at the end of each day's sculpting then when you are "done" and have baked your figure go back and look at each days photo and you'll see the forms becoming clearer and sharper. Taking breaks before finsihing any project is a plus because your eye will be able to see imperfections when you go back the next day that your eyes will have learned to "ignore" while looking at your sculpt the day before. Also having a mirror at your sculpting area is helpful to look at your figure in reverse. This will let you see crossed eyes, lopsided cheeks, one arm shorter, etc.