I'm sure many people have seen this flowchart image before, but since I'm at this stage of my medical school career I figure it was worth re-posting (courtesy of hvattum.net). I'm pretty lazy, and sometimes the light hurts my eyes, so.... hmm... what's really sad is that this chart seems as a good a way of picking a specialty as my last 2.5 years of medical school experiences. D'oh.
For more detailed advice, here are some books you may find of interest:
Another one worth browsing:
Beyond just reading though, I recommend finding upperclassmen, residents, and attendings who share your interests. Talk to everyone: the people closer to where you are in your training can tell you the right steps to take right now to meet your goals, while the ones further down the road (like department chairmen) can give you a big-picture assessment of the field and where it is headed. After that, it's up to you to figure out if you want to go down that road yourself, or blaze a new trail altogether different!
Random Note: Today is Pi (3/14) Day!