My own personal technique is to look at the photo and decide what I like and what I don't like, but then when I post I talk about the bad stuff first, and end on the good stuff. The idea is that even if I accidentally rub someone the wrong way with the bad news, hopefully the good news at the end cheers them up a bit It can sometimes be a challenge to find the good stuff, but there's always something. A whole bunch of smilies sometimes helps too

Of course, some people simply don't want critique, they just enjoy sharing and pats on the back, so try and be sure that they actually want constructive criticism.