my idea of invest now is that I ask myself do I need the money in the next few years? If no, I don't care if it goes down, I'll just wait for it to go back up then sell it at a later time for more than I bought it for. If I did wait to buy however: first, I'd be missing out on some dividend payments; second, I won't know when it is at it's lowest until it's too late to jump in and third, I haven't found a reasons to fear a market drop because I don't need the money right away.
This said, I kind of dollar cost average mostly because I don't have enough to lump sum it, I put money into 401k (into vanguard target fund, it's a 80/20 split). I front load my roth ira at the start of the year and twice a year I put it about 5k into taxable account to make the fees reasonable since I use scottrade.