How to cite a conference paper in an "extended" journal version?
I write something along the lines of:


  This paper is an extended version of work published in [1]. We extend our previous work by A, B and C.


Note that some new figures etc. will probably not really count as new content (this would probably be called new presentation rather than new content). New findings and new data, that's what people will really be interested in.

One common pattern that I often see in my field is that the conference paper has most of the ideas in place, but a rather cursory evaluation. The journal extension then has the ideas of the conf paper + whatever input came out of discussions at the conference + a more or less large-scale validation. The "nucleus" of the paper is reproduced more or less in verbatim. To be fair, I am not sure if this is even ok from a copyright point of view, but it certainly is common practice in my field FWIW.