Consistently invest an amount that you are comfortable with. It's called dollar cost averaging. I'd personally keep dry powder for when the market is far beyond the standard deviation of historical records. 2008 and 2018 would both be years this is true. Shorting though is only for the brave or stupid.
Personally, I put a certain amount in every quarter.
Or it is wise when you allocate 5/95 split to stock/bond but fool's errand when it is 0/100