I'll lay out my current philosophy here.
As far as my 401k, I contribute each pay period, and let it ride with a pretty aggressive risk allocation. No timing, don't care.
For my IRAs, I contribute when possible, but I don't invest unless the market has dipped quite a bit recently.
As far as investments that I've already purchased... BUY and HOLD.
For the current market environment, risks are present as always, but I don't think I've been this confident in stocks for the mid-term (not greedy, just confident). I want to limit trying to time the market as much as possible, but I rarely buy a hot stock.