This is a great question--and one for which I don't know if there are data. This is particularly important for students considering residencies that factor in CK scores in evaluating applicants.
My school does not have any specific program to help out MSTP students for Step 2 CK. Much of the clinical knowledge is acquired during 3rd year clerkships. CK focuses mostly on diagnosis, with less basic mechanisms, and a bit of management. There is a hefty amount of medicine, OB/GYN and pediatrics on it. I prepared for it using the tools most other students use--USMLE World, the usual review books, etc.
What really needs to be jumpstarted is familiarity with the clinical world and some commonly encountered clinical problems, expectations on the wards, and what exactly you are being evaluated on. Our medical school does have an "introduction to wards" type 2 week program for all 3rd year medical students starting clerkships. This was quite valuable, but not necessarily geared toward MSTPs. In addition, our MSTP program has medical sciences grand rounds in which clinical cases are presented, and encourages us to do a longitudinal preceptorship with a clinical mentor during the graduate school years, in order to maintain some contact with medicine. Some students also do one clerkship before they start graduate school, which gives them some clinical exposure earlier. Although all these things help, the transition is still very difficult and the perfect solution has not been identified, at least at my program.