I got my Master's license five years ago, when I realized that I had enough sea time on my own boat (360 days in the previous five years) to qualify. At the time, I decided that the difference between the OUPV and Master's requirements wasn't that much, and a Master's sounds more impressive.
I studied up from a guide I got at West Marine - I think it was Gonder's book.
I also used a program called NavRules to drill myself on the Rules of the Road - this was the toughest part of the exam as there are many arcane details and you need to score 90%. (The rest of the exam is open book and 70% is passing.) There's a link to the program below (but it didn't work for me just now).
I was lucky - I passed the exams the first try. Got exactly 90% on the Rules of the Road part. I finished at 11:30 am, so instead of going for an early lunch I asked for the towing endorsement exam and passed that too. But then, I enjoy taking tests and have something of a knack for it. I also have a commercial FCC Radiotelegraph Operator's License. Don't know if anyone uses them any more but I keep it renewed.
Good luck.
Walt Bilofsky
Master, Inland Waters, 50 Tons
bilofsky@toolworks.com