How to choose a site to upsell/x-sell?
 

I was thinking about upselling/x-selling sites from a pay site. Is there any strategy how to choose a site for it? Should a site, I will upsell/xsell, be in the same niche with my?
What are the hidden dangers here? Any advice will be appreciated.
Thank you.

I thought somebody has some tips here. Well, do it my own experience.