A Time to Rebalance Portfolio

I was reading WSJ article A Time to Let Go Of Overvalued Stock and it got me thinking. While I don't have individual stocks, the recent run up on the stock market has thrown my asset allocation out of whack!!! I don't think I should rebalance my portfolio every time we have market run up, but with my equity vs bonds ratio is close to 4% different that my target allocation, I decided to rebalance my portfolio. Earlier in the year, I actually have been doing rebalancing regularly, not through exchange, but by investing my Roth IRA contributions toward funds that have been lagging. But since I have reached my contribution limit for my Roth IRA for this year, I can no longer "rebalance" through purchase of those funds that have lag the other funds, thus I have to do it through exchange.

I know I probably don't have any readers that follow my blog yet, but in case if you stumble on this blog, how often do you rebalance your portfolio? If you do it once a year, do you stay the course when your asset allocations seems to be way off your target allocations, either due to market crash or market run up? I would like to hear people opinions on this.