It's time IN the market, not timING the market, KatKid. Forget not about dividends, wise one. Having said that, I regularly fantasize about doing some brilliant market timing in my 401k myself.
I tried it once when I was convinced the market couldn't rise further and ended up losing out on enough gains (over months) to buy a really nice car. Got back in, but those losses are still compounding, amigo. I'll take it to my grave (urn, actually; fortunately, this seems to be the one mistake my wife has forgotten).
WWJBD (what would John Bogle do)? Set it and forget it, methinks.
Edit: Keep in mind that your 401k provider surely has frequent trading limitations, though it doesn't sound like you're gonna go wild with this thing.