Passive investing is a little like virginity. You're either passive or you're not. There's no halfway. I've received many letters in the last year from readers who are kicking themselves because they thought the market was due for a decline or a crash and sat through 1995 in cash. Now they are totally paralyzed.
Extremes are never good choices in investing.
As prices rise you will need to rebalance and shift money out of stocks. Another route, if you absolutely can't be a passive investor, is to set a percentage that should be stocks and a range. In other words, decide that you will usually be 75 percent invested in stocks but that you can sell down to 60 percent when you are worried and buy up to 90 percent when the market looks undervalued. Note that neither of these methods requires an accurate crystal ball. It allows people who don't have enough money to hire an investment manager to own shares in a pool of money that IS managed by a professional.