This falls more into the "tips and tricks" category, but I wish someone had mentioned this to me earlier. Once you have a good book on C[1] install all the man pages for the standard library. Once you start writing code, being able to say something like "man strlen" is amazingly useful.

I made the most progress in C when I took a class on OSs. Mainly, we wrote systems programming sorts of things like a simple shell, our own version of malloc, a simple user-space thread library, and so on. This really gave me insight into how these things would be done for real. Also, writing something really linked-list-heavy gets you a feel for pointers really quickly.

[1]: I have K&R ANSI C and I found it very concise and well-written. It is probably one step up from the "raw newbie" class of books in terms of difficulty, but should feel just right for someone already acquainted with (an) other language(s).