As someone who started "lifting" last year on an inconsistent schedule and only recently got on a 3-4 day schedule for lifting, I think discipline is key. I never look for motivation anymore because it just leads to procrastination. So basically, make it a habit early on so even in med school, with the great amount of stress and studying, you can still pump some iron.
Also, I just want to ask the experienced lifters: do you recommend protein powder? One of my friends who's made incredible gains says he take protein powder after every session and also creatine. He was a lanky guy and now he's got ridges and ripples all over. IF you do recommend it, any idea on how to not pay $50+ for a 5 lb tub?... it seems a bit excessive