I bought a musician's metronome that clips on my ear. Right now I'm running indoors on a treadmill, but when I go outside I'll have worked out what pace to set the metronome at to run my 11-minute or 10-minute miles (I'm slow ). I plan to set it to beep every 5rd step so I don't weight my run on one leg more than the other, and also because that's how I try to breathe to prevent cramps in my side.
I also use it indoors on my bike trainer because I don't have a cadence computer and also it allows me to watch a movie without breaking pace.

-- Joolie