Prepare yourself to be as helpful as possible. The way you interact with someone else's work can affect how they see it themselves. Treat it with respect. Remember that you are working with someone to make their project stronger. It is not you versus them - it is us working together to create the best version of the work possible.
Ask for permission and direction before giving feedback on someone's work.
Write on the work to show any feedback and post it for everyone to see.
Start the critique with a smile - and maybe even make people laugh.
Make it a joint venture between yourself and the person whose work is being critiqued.
Take the time to develop some strong opinions about the work and share them.
Ask who the target audience is for this project and how they will experience the work.
Call attention to what is working well and build off that success in other areas of the design.
Identify and assess the use of a visual language in the work.
Notice and share your first impressions.