Bottom Line Up Front
I'm not sure where this phrase originates, but musicians have been doing it for as long as I can remember. It's a way to make sure that when we finally come to the end of our piece or song, we're not caught by surprise.
It really isn't any more complicated than that, but it takes a surprising amount of discipline to make sure that we actually do it. It's so tempting to start at the beginning to try to do the run-through each time we approach a song, yet that isn't necessarily the most effective strategy, and often we can find ourselves struggling at the end.
For that reason I like to primarily work off of charts while I'm work-shopping a song. That way, I can literally turn to the last page and work my way backwards through the piece. It need not be actual paper charts, though I have to admit that that is my personal preference.
Lead sheets, especially those that do not have the verses beyond the first verse integrated into the music, can pose particularl challenges to this practice. In the past I've resorted to writing in the lyrics, though lately, I'm more likely to learn the final verses and outro by wrote, and then go back to the first verse.
However it works best for you, make sure that you're working the end sections of your pieces as much as the front half. This helps us to make a strong finish and propels us into the next song with confidence.