What's your favourite gear ratio?

Yeah, about 205rpm.

Learning to spin without bouncing takes time, but you can smooth things out a bit in the meantime by doing what CJ says above. However, it's a bit esoteric - how do I do that whilst riding? - so here are some simpler steps:

• drop your saddle 5mm
  
• concentrate on unweighting your feet on the upstroke - feels a bit like trying to swiftly bring your knees up towards your chest
  
• do some progressive spin-ups: I can't just jump on my bike and hit 200rpm without wrenching something; quick 5 secs up to perceived 95% max cadence; repeat after a few minutes; by the 4th or 5th time you do this, you're perceived 95% max cadence will be ~10-20rpm faster.
  
• push against your bars without locking your arms. This will help stabilise your hips/core.
  
• relax
  
  

These sound like good tips, I'll have to try this. I try to think in "circles" rather than up and down with the pedaling motion, find this helps you bounce less and seem to be able to maintain the spinnyness for a longer period.