how to present a talk on work in preparation I guess this is very common, to do a presentation to your lab. I wonder how I can cite in my slides that the presented work will be part of a future paper.

I remember to see in past presentations a footnote with " Author A &amp; Author B (in preparation)", but I am not sure.

or I should forget all this and just present the work?
If the work corresponds to a paper submitted to a conference, I put in the slides "submitted to conference X". Otherwise, you can put "ongoing work with Author B". If you present the work in part of a seminar, you can put "Seminar of the department X". 

But mainly, you don't have to put anything, especially if the slides are not put on the web or distributed. The only important point I would say, is to indicate if your work has been done with other people, in which case, you can write "Joint work with B".