Do lots and lots and lots of problems until you are sick of doing problems, and then do some more... There is no easy route.
I utterly concur. The choice to give it your all is most important.
Also, I wish to add that staying focused is also fairly crucial. I didn't let anything distract me from my purpose, and I made a heartfelt commitment to giving my absolute 100% on every single assignment for every class that I took, no matter how small or petty it seemed to me. I was very honest with myself, listened to any useful feedback, and if I slipped from my commitment, I immediately recommitted. Often this meant very long hours going over dry stuff over and over again until the sun came up in the morning, even when my eyes were burning and even when I could have be doing something else more fun like sleeping.
I generally stayed with what I was studying until I could teach it to somebody who had zero experience in the subject. And when I accomplished that, I tutored my classmates and proactively set up study groups. I designed practice tests for myself and my studymates well in advance of the actual tests themselves and tried to anticipate what was going to be on them; I designed those tests to be as hard as possible.
I made it my passion to know everything that I reasonably could about the subjects that I studied. I never expected my professors to teach me anything, and when they turned out to be great professors, which they frequently did, that was a great bonus. Basically it all comes down to being willing to take 100% responsibility for your success and working in partnership with your professor.
I also made a practice of letting go of any self-sabotaging habits I still had. I didn't care that almost everyone around me thought I was crazy; I was on a mission and I had a vision of how to get there. I let that vision guide me when I felt scared and alone. Have people around you that understand where you want to go; it helps to have a cheering squad handy for those times. Invite your all of your friends to give you honest feedback about your process. And, oh, yeah, make sure to learn from every mistake that you make.
Of course I wanted to get all A's, but I never let my grades be my primary focus. That would be falling into the pre-med trap. For me it was about doing my honest best at all times, learning the most that I could, staying in good balance, giving away whatever I learned freely, and being of service to myself and others when I could. The grades were orthogonal to my approach. Doing well in school (being a good student) is probably necessary and beneficial to the admissions process, but isn't anywhere near sufficient in the process of making a good doctor; keep everything in good perspective. Focus on what's important to you and don't compromise anything (except maybe sleep), particularly your health. Stay healthy and schedule regular exercise. You can't do anything if you aren't whole.
Best of luck!