For the person writing a critique, it does no good to send a "Squee! Squee! Squee!!!!1!1eleventyone!!1!" email or instant message to the writer unless there is nothing that needs tweaking. And continually doing this, and letting it go on to the point where the writer submits something full of things you should have caught is like letting your favorite female walk down the aisle on their wedding day in a dress that makes them look three quarters dead, with makeup on that makes Tammy Faye's look restrained, the back of her dress tucked into her pantyhose and trailing toilet paper from her mismatched and dirty Keds. Seriously, if you think you're doing someone a favor with nothing but constant praise you're dead wrong and not helping.
On the writers part its a bit more complicated. You have to mentally tough enough to look at negative feedback and decide; A) Is this really a problem? B) Do I know a way to do this better or can I find or devise a way to get the result I want? C) How much do I really care about this particular item? And D) What other mistakes like this have I made?
And on picking your readers; send them something you KNOW is unmistakably bad every once in a while. Anyone who doesn't comment unfavorably about such things consistently is no better than a glorified breathing spell checker. And of course, you need to know what each of your readers strengths is. The person who does your line edits and says stuff like "What the hell did you use THAT word for?" may or may not be you best source of information on 18th century military uniforms Likewise, the math geek who crunches numbers and answers your questions on projectile weapons and mass to energy conversion may just be completely clueless about what the difference between a SSRI and a Tricyclic antidepressant and how either or both of them compare to Monoamine Oxidase Inhibitors. This is fine, actually its probably better to have varied backgrounds among your readers. This way when people who start commenting on things they never before mentioned you know that something has changed and this should be the signal to figure out if this is a change for the better or worse. Unless you’re up against a strict and impending deadline it can’t hurt to take a couple days between reading the critiques for the first time and reopening the work in progress to internalize the info and let some of the sting go out.
The bottom line is; don't waste time and energy. If you're giving a critique, give one that counts. If you're getting one evaluate the information for what its worth, apply what you think is prudent and or needed and move forward.