Exactly what the Subject says.... How often do you rebalance and why?
I rebalance "passively" (not sure if that is the right term). Since I fully contribute to my 401k, I typically don't try to buy and sell so much of my existing holdings as I just change the allocation of future investments. Based on the current limit of $17.5k/yr, it's around $1.45k/mo in contributions.
Recent example, small caps have been lagging this year, so I've upped the percentage of my contributions going to US small caps and intl small caps from 10 to 20% of contributions. I will ease back to the "base" of 10% once the % gets closer to the right mix in my overall portfolio (includes holdings outside my 401k). The same idea get applied to other asset classes too.
In case you're wondering, my securities overall targets are 50% US stocks, 30% Intl stocks, 15% Alternatives, 5% FI (I'm 25, by the way). Right now, total portfolio is around $140k, with ~$100k in equity in an investment property. Within each category, I don't come up with specific targets, but try to keep a mix as best I can. Once my portfolio gets large enough, ($1m+), I may need to rethink how I rebalance, since shifting my monthly 401k contributions may not be enough. That will be a while, so I suppose we'll see.