Everything is important, but once your GPA is above the screening cutoff (3.3 or so to be really safe), you can start worrying about other things. Programs look for a well-rounded candidate, although what that means may vary from place to place.
Professional involvement (not simply signing up, but actually doing something), hospital work experience and community involvement are all big factors for most programs. The academically based programs look specifically for research experience (posters, publications, etc.), but all programs view this as a positive. Again, the weighting of these factors will vary from program to program.
Letters of recommendation are an very important. You'll want to get these from people who know you on different levels, so work supervisor, professor and research advisor is a nice mix to shoot for. Make sure your letter writers have written letters before, as these will frequently carry a lot of weight.
Potentially the most important factor is presenting yourself appropriately. Have someone who knows what they're talking about take a look at your cover letter and CV, as poorly written versions can kill your chances. Keep a running tally fom your P1 year about everything you've done and continually update your CV. It's very easy to forget about things when you're a few years out.
It's a tough market, you should try to do everything you can to separate yourself from the pack.