Read code, write code. Repeat.
Here's the faq for the alt.comp.lang.learn.c-c++ usenet newsgroup.
http://www.faqs.org/faqs/C-faq/learn/
From their faq:
14: What are the best books I can learn C from?
If you wish to learn C, the classic text - the "Bible" - is "The C
Programming Language", 2nd Edition, by Brian Kernighan and Dennis
Ritchie. This hallowed text describes and explains ANSI C. K&R2 is
renowned for its brevity, clarity, elegance and completeness; but
these very factors can make it heavy going for the beginner.
http://cm.bell-labs.com/cm/cs/cbook/
K N King's "C Programming: A Modern Approach" is another text
frequently recommended on comp.lang.c. This book is a good,
thorough introduction to C that is a lot easier to work with
from a beginner's perspective.
http://knking.com/books/c/
Another frequently recommended book on C is "C - How to
Program", 2nd Edition, by H M Deitel and P J Deitel. Please note
that I have not had a chance to read this book, and include the
reference on the strength of the the recommendation of a number
of regular posters to comp.lang.c.
http://www.deitel.com/products_and_services/publications/chtp2.htm
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Deitel & Deitel's books are pretty good - I had a class on Java that used their Java book.