A lot of people think that they are what they do ... this leads to great sensitivity to criticism - but let's face it - no-one, period, wants to hear that they are not perfect. They know they're not, but they don't want you to know ... silly maybe. Criticizing helpfully first means only work to their strengths, balance your criticism with praise - everyone has good things and bad things. If you are solicitous of them they will accept the negative things, and what criticism isn't negative, in the positive light that you are offering it.
You might offer to help out. The point is to be life giving and not life draining.
-- RaySchneider
Sometimes it helps to use the word "and" instead of "but". As in, "You're doing great, but you need to change this". If you use the other word it doesn't sound so negative. -- DaveHarris
I took a workshop once that advocated a "liked best ... next time" pattern to giving feedback on someone else's work. What I like best about this is that it not only affirms what they have done so far, but it also acknowledges that the work is a series of improvements. The next time the come up with a pattern like this, they should also think about how to handle cases where there is to be no "next time". -- WaldenMathews
I take the approach that if you do not offer guidance beforehand, you have no right to criticize after the fact. If you let someone go out and do something on his own, unless it is an absolute failure, just let it go. If you know a better way, teach it or recommend it ahead of time. No one likes to spend time on something and then have a casual observer explain how it could have been done better. -- WayneMack
Helpful criticism is criticism that helps, which implies some greater objective or purpose other than a simple dissection of strengths and weaknesses (ie. criticism for sake of criticism).
The difference between CriticizeHelpfully and CriticizeUnhelpfully? is that the latter, although it may point out matters of fact, it doesn't point out matters of fact which are relevant to the bigger picture. Offering criticism on a winning game of chess is helpful if the criticism reveals weaknesses which might lose the game, but criticism that points out that the little horsies are facing different directions isn't helpful. Criticizing the content of this page by noting the fact of the (in-)correct spelling of the title word is unhelpful.
See also AskPermissionBeforeCriticizing, CriticsAreYourBestFriends