I hope I always remember to begin every critique with something positive. That's because I've never seen an image that doesn't have something worthwhile in it and because noting the positive characteristics is as helpful as noting the negative characteristics. I wouldn't want a positive characteristic to go unrecognized for fear that the photographer may have made it happen by accident or perhaps doesn't fully appreciative how important it is to repeat that characteristic in future photos.
As for then following up with information about characteristics that could be improved, I try to remember to offer up a suggestion for the photographer's consideration. That is as opposed to implying or stating that whatever I'm recommending is such a great idea that it should not be questioned.
Considering the amount of critique that you and I have shared, if I ever veer from the above, please slap my wrist through your monitor and get me back on track.