Other answers have already stated the general idea, but my advice is to hew to the fairly clear, specific principle: Will my comment add some new non-trivial foreground or background information to the discussion that hasn't already been brought up?

Or, put another way: opinions don't really matter; we've all got one. What matters is bringing specific new information to the discussion which could make a difference.

(Oops: I see akg_67 has just brought up this same point while I was adding this. Oh well, I suppose I should delete this to be consistent, but perhaps two similar answers will help drive home the importance of this point. ;-)