<p>Try The Art and Craft of Problem Solving by Paul Zeitz.</p>
<p>Also try the Challenging Algebra and Geomtery books by Salkind.</p>
<p>Get lots of old AIME and AHSME tests.</p>
<ol>
<li>Prepare, like everyone else has said, by reading The Art of Problem Solving volumes I and II. If your goal is to do well on the AMC series of contests, like make USAMO for instance, then you should do as many old AMC's, AHSME's, and AIME's as you can. Other books that might be useful after Art of Problem Solving (keep in mind, these are getting to the olympiad level) are the Art and Craft of Problem Solving, by Zeitz, and Problem Solving through Problems. The Mandelbrot and ARML problem books have lots of practice, while Geometry Revisited would be a good selection for advanced students who want to tackle olympiad-style questions.</li>
<li>My first contest was in 4th grade at age 9, but I didn't start preparing in an organized fashion until 7th grade. I didn't start working on my own (which makes a huge difference) until sophomore year.</li>
<li>Depends a lot. When I was concentrating on math during the summer, it might have run past 4 hours some days, with at least 1-2 hours on other days. On the other hand, most days I don't prepare.</li>
<li>Yeah, sometimes you feel like its taking forever to get better. For example, when I was grinding through all the AIME's last summer there was a time when my score just stayed about the same and even dipped lower. The trick is to think big picture- once you reflect over a couple of months, it's clear that you're improving.</li>
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<p>Also, I second the idea that you must love math. Although it's possible to push yourself for some external motivation, it will be so much easier to do work and to feel good about yourself if you enjoy the subject matter.</p>