A few years ago I had an English teacher that encouraged "The Oreo Method"; it compares effective constructive criticism to an Oreo cookie.
The filling in the middle was the constructive criticism, but before and after that, you offer positive feedback for the writer.
Pretty self explanatory:
1. Provide one piece of positive feedback first and linger on it for a couple sentences; let them know how important that "thing" is and, in a way, praise them for doing it. This primes them to accept your feedback cause they know how thoroughly you've read and analyzed their work.
2. Offer any and all of the constructive criticism you have; stay subtle and be concise with all your feedback.
3. Offer more positive feedback, as many good things as you can come up with.
By submerging the constructive criticism between positive feedback, you keep their hopes up while still thoroughly conveying weak spots in their work.
I hope this kinda made sense; it's a very self explanatory idea so I had trouble elaborating on it.
