My son Ben passes on two suggestions:
First, I found this great free Timer application for the Mac. You can use it as a stopwatch, to countdown, or as an alarm clock. For instance, I can set it to run for forty minutes; when that time is up, the application starts up my iTunes visual screensaver and I know it’s time to take a break. I do the same thing to time my break. Here’s the link: Apimac Timer.Thanks, Ben!
The second tip is a little alteration of your 45/15 rule about studying. I’ve found it’s fun to increase the amount of time you’re studying and reduce the amount of time you’re taking a break each time. So for instance, one of my first sequences ran like this: 50 minutes studying, 6 minute break. The next one was 55 minutes studying, 4 minutes taking a break. It’s a way to increase your productivity in a gradual way, and it’s very easy to do with the Timer application.
A recent post at TUAW will lead the curious reader to a variety of Mac (and Windows) timers.
A related post