Practice is not enough in my opinion. In chemistry lessons, do you just do practicals? How much do you learn from mearly doing practicals? There many ways to improve and get artistic knowledge in your head; participate in workshops, buy books and learn anatomy (Gray's anatomy is a good one), take lots of photos as they are essential for reference and read tons of books on technical stuffs for art (as mentioned in my long list above). But what I consider the most important thing would be experimenting and discovering your style.
Even though mearly practicing is not going to get you anywhere, practice is key. For example, when you read books about anatomy, sketch out the particular part of the body you are observing on a piece of paper. Whenever you learning any aristic thing, practice and attempt the thing you are learning. Practice everyday, makinga gazillion sketches on a piece of paper and using loads of paper. One piece per day is not good enough. Your paper should look like this (not my art, I just got a random thing off dA). As you can see, there are lot's of experimentations of a single subject, not much time was spent on each sketch.
http://imaginism.deviantart.com/art/Sea-Dragon-design-exploration-127680826
Now that I've said what I think is a good way to learn, here is what I think about your sketch:
Pose and junk- listen to WretchedHelios
The head doesn't look right at all, it's slanted (if the head was a square, this would be a rhombus). You should see the shoulder of the other leg. From what I can see in your picture, it's supposed to be a realism piece.
What my rules are for realism pieces (I don't do them often though):
- Use several types of pencils (preferably B-6B)
- Don't use lines to define shapes, use shading and tone and stuff.
- Use a rubber, not for just correcting mistakes, but for adding highlights
- Use several pictures for reference.
Now scribbling on it like how my ex art teacher (I don't take I/GCSE art), Mr. P would. Here's what I think you did horribly wrong by using LINES to define the shapes and next to it, how I would shade it (I only did a little bit). I used photoshop to do that, so it looks funny.
http://i246.photobucket.com/albums/gg100/HappyVeggies/dogajkshdkajshdkaj.png