The reason that this is a problem is that the average person has more friends closer to the median IQ than they do further away. So when people share new sites with one another they are generally sharing it more often with lower IQ people than they are with higher IQ people. These people come, love the content, stay, and repeat the cycle.
Countermeasures are just fighting this effect. It happened to Reddit, Fark, etc. One countermeasure is smart-flight evidenced by people fleeing to other subreddits like "TrueReddit" or making nearly identical subreddits "CanadianPolitics" vs "Canada". Or "LibertarianDebates" vs "Libertarian". It's like "white flight" only the division isn't racial, it's cognitive.
In my opinion HN suffers far less than it reasonably should, given its popularity and influence. Probably due to good attention, moderation, and direct involvement by pg. It's likely that when I joined HN I knew far less about the world and tech; upvoting the 500 mile email posting or a basic article on split testing. But I've grown and the community probably became a bit more mainstream and less intelligent.
The only community I haven't really seen it happen to so far is Less Wrong, but over there they are absolutely fanatical about keeping out poor discussion. I don't know if that is worth the effort.
To me, the best thing to do is to just leave once it gets bad enough and a better alternative pops up. Something else will come along. Maybe a secretive invite only HN, or something that customizes which articles you see based on what you upvote. The problem with the former is that secret societies are generally self-important and word always gets out (remember fight club anyone?). The problem with the latter is that it absolutely ruins a feeling of community. So those probably won't happen, it will probably just be a new community, much like this one, but with a clean slate.