I was more active from early '07 to '09 than at any other point. In late '06 I got concerned both about housing and about turning 50 in two years while I had a 90/10 allocation--so I sold stock mutuals for about a year from mid '06 to '07 and bought bond funds to get to 65-30-5. Then in Oct '07 I got concerned about a crash and the fact that the bond funds I bought were an intermediate (corporate and goverment note), so changed 70% of the bond allocation to govt intermediate (the note fund went to PIMCO Real Return).
Then during the crash, in early '09 I bought small positions in Canadian funds, China, gold miners, and some more of my core funds, but it was less painful to just increase my monthly purchases more to stock funds. At the end of the year, I started moving the intermediate fund to TIP, a high yield, and floating rate fund, and a foreign market bonds and bought back China and biotech.
I was lucky--most of those turned out well, save for a early (and luckily) small move into an Energy Services fund that was creamed.
From then until 2011, I pretty much just let things sit, but sold some of the stock fund gains in 2012 and 2013 to stay close to my 65-30-5 allocation.
Rebalancing in a crash now, without the monthly contributions will be much harder, I think.
I did manage to rebalance in 2008, early 2009. It was very tough. I remember looking at my portfolio in Jan 2009, and the only way I was able to rebalance was because I could see I would have 10 year of expenses still in fixed income after rebalancing. That was the only way I could bring myself to do it. And the market took another nasty dive into March of 2009! Fortunately, that was the bottom.