Prepare by solving lots of hard problems, that's all you can do, I hear that Irodov is an excellent source of problems. This assumes you've learned the material that is covered in the competition but If you haven't done that then that's the obvious first step.
Study as much calculus based physics as possible. I got into the semi-final round for the US Physics Team in 2000, but didn't get past the final round of selections. I think I read Serway's textbook about a dozen times.
I have a blog that contains some physics problems and solutions. I think its good to improve your physics skill. Also, I have a download link to some IPhO problems on the same blog.
Here is the link of my blog: http://collectionofphysicsproblems.blogspot.com"