This comes around about once a month on average. My method is to ponder the subject at hand for most of that time and then waiting to the end to put it to paper. If I don't let it "cook" long enough, I end up re-doing it.

The way I know it's ready to be written is when my mind drifts to next month's subject rather than dwelling on the one at hand.

This approach can be unnerving until you get used to it, but any more I wouldn't do it any other way.