Google for "Style Guides" and read about it. The style guides help you to code more effectively. And switch to better compilers. I know a few people who are still stuck in TurboC even today and thats really bad. Learn about make files, cross compilation, etc. If you feel bored then learn something new like a graphic library, network programming perhaps or maybe how to use regular expression engines within C. All these help improve your C skills. Also I'd recommend that you take up some projects and try them preferabbly using a VCS and host it in google code, codeplex or sourceforge or some other such forum. This way you can learn a lot from others and learn how to work as a team as well!
Study good code. There are some great examples out in the open source world. You should visit the gnu.org web site - all their tools have source available. Start with small utilities at first, and try to understand what they are doing to implement the functionality that the tool provides. Then, try writing some of your own tools, keeping things simple at first. From there, work your way up until you can confidently handle more complex tasks.