There are obviously many theories on this, I found one that to me makes good sense. The first choice is #4 hitter, highest slugging percentage hitter goes there. Then #3 and #5 hitters, the next two highest slugging percentage hitters go there. Then #1 and #2 hitters, highest on-base percentage hitters (preferably guys with speed) go there. Then #6 thru 9 spots are filled with the remaining hitters in order of decreasing batting average. I personally don't buy into the #9 spot being a '2nd leadoff hitter' theory, to me it seems like you'd have a better chance when your best hitters are at the plate more times in a game, and the #9 spot is going to bat fewer times than anyone else.