Just finished residency, took boards and luckily passed. As for studying during residency...sounds cliche..but study from your patients . Read about your patients and learn what you can about their disease presentation and management, etc. Teach your med students and junior residents what you know, you learn from reviewing as well. Make a point to go to your residency didactic lectures or case presentations, its easy to blow them off to try and finish up your notes or whatever, but your in residency to learn and lectures are a part of that. My residency was good at covering the topics on the boards and most programs make a concerted effort to teach you what you need to know. How much you remember is a different story. Dont be afraid of the in service examination, its a good time to practice and find your weak spots. And for review, I agree with Laughing Your Way Through Boards. Excellent review, sometimes too corny but it got the job done for me. PREP questions helped as well, but you have to read the answers. My program gave us like 5-6 questions a week and offered a raffle for everyone who did them so I would usually do them.
As for reading Nelsons cover to cover. I obviously haven't done it but I think its about 1200 pages long. Divide that up...ever try to read 35 pages of Nelsons when your on call? Post Call? Anytime? Its brutal. I think its a reference to read on certain topics.