I started with a Garmin 910XT, which I originally bought as a backup GPS for hunting in case my main GPS stopped working. It has an interval timer function that gives you a little buzz on the wrist at the end of each interval. The shortest interval available on the Garmin is 10 seconds, which gave me time to breath, exhale, draw, and hold. I think there are many other similar athletic wrist gadgets similar to the Garmin that have an interval feature. Finding out how short an interval they will support in advance of buying one is a challenge, however.
I prefer a shorter interval, like 2 - 5 second intervals rather than 1 - 10 second interval, so I looked around for something else. The iPhone would be ideal, since I carry it around all the time anyway, and there are all kinds of metronome apps available for it. The one I use is called Tempo. Unfortunately, the iphone buzz is too puny for me to feel through my pocket, and I don't want an audible sound that will bother everyone else. The iPhone metronome works fine through a wireless earpiece, so a lot of times I'll use that. If I'm just at home practicing by myself I'll just carry the iPhone in my pocket and use the audible beep. The weakness of the iPhone buzz is also a problem for people in meetings, etc., as they miss their calls, so maybe Apple will improve this in subsequent models.