Bouncing at high cadences

How do you measure your rpm? Do you actually count it or have a speedo device thing? The bouncing happens to me a bit. I might try all the smooth circles stuff but up to now I've just been bracing my arse into my saddle - works ok, hard on my wrists and arms though :/

Can do it with a cheap coomputer to measure your MPH, then check that against that rabbit gearing/cadence chart.