How should I cite presentation slides? A friend has made some nice slides that I could reuse (similar topics). He sent me the slides and commented that if I use them and could cite him that would be nice, I asked him how should I cite the slides but he said that whatever suits better to me he said "Just add my surname in some place where it's not very intrusive".

I'm not sure if he doesn't care or he doesn't want to be too picky, but I'd like to cite him, to each one his own.

AFAIK, they are related to a paper (but not in the paper) and to his thesis, where they could be as a diagram but definitively not animated. The slides (as such) may be available at some URL, he said they will be but they are not available yet (so I don't have the URL yet). If citing by the URL I guess I could use this: "How to cite a website URL?"

Should I cite slides? If yes, how?
There are two practical purposes to scholarly citations:


acknowledging scientific contribution of others, the borrowing of ideas (mainly) and content (sometimes, in the form of quotes)
helping people find relevant content if they want to read it
bookkeeping, for scientometric/bibliometric purposes


In your particular case, you cannot fulfill #2 and #3, because your friend's slides are not available for others to read, and even if they were, random documents on the internet are not really used for bibliometric purposes.

So, you want a solution that will achieve #1, i.e. make sure his contributions are recognized by people who will read your slides. To do so, you don't need to give your citation any specific format. I suggest you simply write, at the bottom of slides you borrowed from him:


  Slide courtesy of John Doe


or 


  Slide modified from John Doe, with permission


In addition, you can thank him in your acknowledgements at the end of your talk.