Just as the title says, I will show you the character developpments that happened between Luffy and Nami since the beginning of the series ("show" is the keyword). I found this cute little comic one month ago on pixiv (don't even know if you can call that a comic) and it showed VERY SIMPLY how the relationship between the protagonist of the greatest manga ever and the main female of the same series progressed.
I felt like I needed to show it because too many people don't seem to notice this. In fact, even I, a hardcore LuffyxNami shipper, never noticed the difference between Nami and Luffy's relationship, post-Arlong Park and around chapter 800. The words are in japanese but I will roughly translate you what it says. First look at the pic.
It's the second pic of the file (I don't own any these pics).
First panel: (pre-Arlong Park)