I highly recommend First Aid for the Wards... available at any Barnes & Noble, Borders, Medical Book Store, etc. You will find many of the commonly used abbreviations, but more importantly it is filled with good tips as you start each new rotation (i.e. what to expect on Surgery, Medicine, Pediatrics, OB/GYN, etc.). It is also a good review for some common disease states.
Also, I recommend purchasing a palm pilot (Sony, Handspring, etc.) or... if yo must... a Pocket PC
There are several nice programs that have medical abbreviations, and I think you will find it invaluable for looking up medications and other references. Seriously, I would use the book money you were going to use to buy those big reference books (Harrison's, Schwartz's Surgery, etc.) and buy some of the Skyscape programs (5 Minute Clinical Consult, Outlines in Clinical Medicine, Tabers/Stedman's Dictionaries would be especially useful for abbreviations as well).
Well, I hope this helps. Good luck!
Grant