It's hard to predict how competitive the field is going to be when you apply in a few years. The trend for the last several years have been more competition, greater fill rates, and more ppl not getting their first choices.
If you can manage more than passes, that will be nice. You're right in that they don't focus on preclinical grades - but that doesn't mean you shouldn't try to get honors - you never know what may become the factor that will give you an advantage over another highly qualified applicant. I also got the feeling that some programs used board scores as a screen when offering interviews. So you may want to try to do well on the boards.
I think research in a PM&R related subject would be useful. Spending time with physiatrists to get a better understanding of the field is always great. (Have time this summer? maybe you can spend a few days with one.) These two things will also allow you to get to know a potential letter writer very well. It's always good to have letters from ppl who know you well.
Electives in related fields during 3rd yr is a plus - there was a thread on this - I think Neuro, Ortho, Rheum, Cards are most often mentioned.
Joining the AAPMR - getting the newsletters, skimming the journals - are all extras and may or may not be worth your money. (can skim journals at the library) You can read the first chapter of Braddom or DeLisa to get an overall feel of the field.
This forum is an excellent way to network and get answers to your questions.
If your school has a PM&R department, might be good to try to get a mentor in the field. The AAPMR website also has a link to finding resident mentors. I emailed one I knew - and had lunch with her.
So, hopefully, I didn't overwhelm you. If you have specific questions - let me know. Good luck and enjoy your summer!!
