1. Have fun, relax, and enjoy the journey. You are GETTING PAID for a select opportunity to learn incredibly cool things about science and medicine, and empowering you to pursue a career that is intellectually stimulating, altruistic, not to mention relatively lucrative in the grand scheme of things.
2. That being said, the clock is always ticking whether you choose to acknowledge it or not, and you would be wise to remember that. I think doing a rotation before you start is a great idea as long as no major life events are going on (e.g. getting married, etc.). Seriously...a lab rotation is not that much work, you will probably work 9-5 M-F...that leaves plenty of time for rest, play, relaxation before you start med school, plus it enhances the likelihood that you will choose a PhD lab sooner. You want to take a vacation? That's what 4th year med school is for!!! And trust me, you will need it after MS3 esp. w/the spectre of PGY1 looming.
3. MS1/MS2: Take this seriously and work your butt off, but only so you can be a good physician and a good scientist. Grades should never be your motivating factor. Work as hard as you can so you can be a good physician, and the grades will follow. You want your options as wide open as possible when it comes time to choose a specialty - don't slack off on MS1/MS2 and Step 1 and end up closing off options for yourself b/c you don't have the grades.
4. Grad School: Read one paper a day, M-F, at least for the first 2 years and reference them with a program such as Endnote. That is a relatively small and doable time committment, but it adds up (>500 papers in the first 2 years) and will make writing your papers/thesis SO MUCH easier, will help you design your experiments better, will help you look like a star when you get asked difficult questions at meetings, etc. And always move your project forward EVERY day - make sure you do at least one worthwhile experiment everyday (no matter how small it is - e.g. getting a ligation to work on something you are subcloning)...you always want to be moving forward, even if the steps are incrementally small.
5. MS3: This is the make or break year of the med school years IMO. Grades this year are critical. You must shine, but it is so easy to do so if you are a normal human being with a decent social IQ. Some people just don't get it, don't be one of those.
6. Personal opinion: Focus on the task at hand. When you are in med school, focus on being the best med student you can be, and when you are in grad school, focus on being the best grad student you can be. Don't get caught up in trying to go into the lab while in med school, or going into clinic while in grad school, or keeping up with the lit while back in med school, etc. People will disagree with this, but I think to be well trained requires a serious time committment - your time is better spent in grad school (your protected research time) learning how to be a scientist, etc. rather than going to clinic one half day a week.
7. Moral of the story: PhD is NEVER a golden ticket, you must work your butt off at all stages of the game. But keep your priorities straight, enjoy the journey, and always remember how lucky you are.