YJ,
I take exception to the previous post regarding obtaining a letter of rec. What Amit describes is the textbook answer to your question. It's the way things SHOULD be, not the way things usually work out. Getting a letter from a DO/MD, at least in my experience, is not as hard as Amit describes. Most are more than sympathetic to the plight of the premed and willing to write a (good) letter of rec after getting to briefly know you.
AMS-007 is correct in asserting that you need to get some prolonged exposure working in medicine before making the decision of whether medicine is the right path for you, however that experience does not necessarily have to involve the MD/DO from which you recieve a letter (athough it helps).
Anyways, if you're applying this year, and it sounds like you are, you need to get moving on the issue ASAP. Check with the AOA, utilize the Web, heck, even open up the phonebook and find all the DO's you can (at least 10 if possible), preferably those that specialize in your area of interest (if you have one). Then send them a letter describing your situation, maybe include a short resume to show your experience/education/etc., and sit back and wait. Perfect your application. Hopefully by the time you're ready to submit to AACOMAS one of your efforts will pay off.
When an MD/DO agrees to meet with you, make the first meeting informal and brief...maybe offer to take them to lunch if appropriate. During that inital meeting utilize your personal skills and get to know them. Come prepared with questions, and if you choose a DO be SURE to know what being a DO means (i.e. READ A BOOK). Then milk the MD/DO for information, and show some passion for what they do. Usually if you show genuine interest and an understanding of the profession the MD/DO will invite you to observe them in action. If they don't, bring the topic up and see if its feasible.
When the time comes to do some real 'shadowing' take the time beforehand to read up on the MD/DOs specialty. Know any relevant issues relating to their practice. Impress them by asking how those issues have impacted their practice. Ask many more questions. Keep them talking. Everyone likes to talk about themselves...especially all those egotistical Drs
When you're done with the first day, ask to come back. After a couple times working with them they should have enough info about you to write a really good letter of rec.
Anyways, I hope this is helpful. Good luck.
-d