Personally, I do it the other way around. I'd make sure ad placement is really already good and then start investing on the traffic, otherwise if traffic came first, that's a lot of wasted potential "clickers" had your ad placement already been properly placed. So, if you want good CTR, make sure ad placement is already at its best (improve as you go and from what you eventually do learn once traffic comes in) in order to ensure that what you've invested for the traffic does not go to waste.
Good luck at improving your site's CTR (if that is what this question is for)!
