Be bold, but obviously courteuous, respectful, and professional. Tell the physicians you're interested in shadowing that you're premed, would like to gain some clinical experience by following him/her (make sure you're not doing this to look good on paper but because you are truly interested in witnessing what being a physician is like), and ask him/her if this is okay. If you're interested in a particular field of medicine but don't know any docs, call up the department at a hospital, tell them your position, and see if they know of any docs that might be willing to let you do this or if you could speak to some. I followed 2 ortho docs just this way; I called up the ortho department here and told them if I was interested in following some docs around. They were nice enough to ask around and 2 agreed. Good luck, and don't be discouraged if some say no. Keep trying, since the ones that say yes will tend to be the ones you will learn the most from anyway.