This isn't a big concern right now, but it will be again in the future.
Increase the capital gains tax rate during periods of 'irrational exuberance' and lower it during recessions.
When there is inflation in asset prices but little actual measured inflation, why should interest rates be pushed up to deal with asset price rises? Raising the capital gains tax rate would push out the speculators but likely not effect most beneficial economic activity (depending on how much the rate was increased).
The problem to this idea is, of course, political. Politicians will never raise any tax and the right wing gasbags on the financial news shows would lie that raising the capital gains tax rate would be the end of capitalism.
Using fiscal policy like taxes to fine tune the economy has gone out of favor since the 1970s, and generally rightly so, but there are usually exceptions to every rule.
I'd recommend giving the Federal Reserve the power to set the capital gains tax rate.
Also, anybody know of any liberal forums dedicated to economics?