Buy and hold? Keep % in cash for crashes?

It seems to me that timing is pretty much a fool's errand - at least for me. I don't understand the billion different variables to spot trends and it seems like there's always an "exception" to why this time is different, etc.

So if I just accept that I can't time anything and the more I read, the more unsure I get about it all, what are the board's thoughts on keeping 10% or so in cash to buy when the market drops to a certain level.

For example, would it make sense to keep contributing every month - good or bad - but only 90%, keeping 10% in cash. Now that presents the problem of not know when it's really hit bottom but at least you're getting it cheaper than at its peak.