There are probably different opinions on the subject, so take what I have to say with a grain of salt.
I would try to get on a service that does a lot of cases, and on which you will be the only medical student. This may or may not be possible, but that is what I would shoot for. I say this because this combination will allow you to pick and choose the cases that you want to scrub on, and will therefore allow you to spend time with the attending of your choice. I strongly advise you to ask around and find out whose name carries the most weight on a recommendation. You may like the sparky new CT fellow a lot, but chances are the crusty old general surgeon will be more known and have more pull. Whomever you choose, I strongly recommend scrubbing with one or two attendings a LOT. Round with them, go to the OR with them, and hang out whenever possible. Bust you a$$, and show what a knowledgable, humble workhorse you are. That way, they will get to know you well and hopefully have a lot of nice things to say about you in a very personalized letter. Spreading yourself out by scrubbing with as many different attendings will probably not yield as good a letter as if you focus your activities with one or two attendings.
IF, you are on a service with fewer cases and more students, just go with the flow. Make sure that you don't become an OR hog. If you can't get to the OR, then you can shine by helping out the residents/interns/students wherever possible. IMHO it's harder to get a personalized letter this way, but might be unavoidable if you are on the chairman's service in July or August.
Note: on the interview trail I consistently heard remarks like, "Dr. X has written you one of the best recommendations I have seen so far." I was the only student on Dr. X's service, and scrubbed on every case he did. Even the middle of the night traumas cos' he would page me if I wasn't on call. I wound up working my nuts off but we got along great and he apparently felt comfortable writing me a kickass letter. God bless him! My letter from the chairman did not elicit any remarks like this however. I'm sure it wasn't a bad letter, but I only scrubbed with him a few times in 4 weeks, and rounded with him 4 or 5 times in that period.
Books: Advanced Surgical Recall, Current Clinical Series Surgery, Maxwell's, and Pharmocopia.
Peace, and good luck!