Part of the reason I wound up on this site is related to my quest for meaningful feedback on my developing writing skills (I'm still well withing my first million words, for those who've read OSC's book on writing).
To this end I've tried taking a couple inexpensive online writing courses and have been disappointed. In one case all the feedback was happy and full of superlatives (I believe I have some undeveloped ability, but I'm not that good--yet), in another it was all about conforming to a set of style rules the instructor believes in (expunge passive wording, only certain types of dialogue tags, etc.).
Aside from (hopefully) this site, what else have all of you found for good sources of truly constructive criticism?
Sometime this weekend I'll toss 13 lines of something up on the forum for such things and see what happens, and check out what others have put up.
Any advice for making the best use of this site, and what I can do to best help others on the site?
Welcome to Hatrack though.
As for you questions, I think you'll find that we take our critiquing very seriously around here. As for guidelines on how to do it, I'll let some of the professionals handle that.
The feedback is fairly good around here, but what you have to understand about those who want you to conform to their own stylistic preferences is that you're not going to get away from that. The trick is to take what you can use and ignore the rest. As long as they're giving you an honest opinion, the critique can be useful if you know how to read it and react to it.
You should avoid passive phrasing, for example. The reason is that this can be boring for the reader. That's why I like Card's wise reader approach. Instead of saying, "Your wording here is passive, you should change it." I might say, "I was bored reading this passage. I think it may have something to do with the wording, because there is enough going on in the scene that I should be interested. Perhaps you might consider a more active turn of phrase?"
But even if they say it the first way, you've got to be able to backtrack and figure out if they're harping on about a pet peeve (see the pet peeve topic...misspelled, of course 
) or is it actually a problem, something that has caused them boredom, disinterest, or disbelief?
There are two ways of knowing:
1. Many people tell you the same or very similar things.
2. Their advice resonates with you and what you are trying to accomlish.
quote:
Aside from (hopefully) this site, what else have all of you found for good sources of truly constructive criticism?
Scrawl, if you can 1) get them interested in your writing (they tend to sneer a bit at 'genre' writing, more fools they), and 2) you can put up with their gleeful encouragement of obscenities, and 3) if you can ignore the 90% of the site dedicated to flame wars and posturing.
http://www.stwa.net/scrawl/index.php
Notebored, but many of them there are also here.
http://rittinger.admiralxp.com/index.php
Critters is a bit of a crapshoot on the quality of feedback, and they are pretty tight about their critiquing X for Y policies.
Your best bet (only IMO, of course) is to develop the relationships here to get into a really tough critiquing group.