There are a handful of players who I've spoken to who have said that one of the successes of the Russian grandmasters is their memorization of 300 key positions. When I first heard this concept, I was skeptical. I have done a little digging only to find out that it has some truth to it. Many Russian GMs have written books and entire manuals dedicated to the '300 positions'. I know the number doesn't seem like much. I first thought "I'm sure GMs memorize thousands more than a mere 300 positions", which is the truth... but the point is the 300 positions usually middlegame positions with many posibilities.
One study has found that modern day Masters can get away with memorizing 10,000 positions, while IM's and GM's memorize around 100,000. In the 300 key-positions, it's possible to come to a conclusion of 100,000+ positions. Now what I mean by memorization is to memorize the position like you would your first language. Say for example, you remember one position as you would the word "hello" in English. You'll never forget the meaning of the word, thus you'll never forget the elements of the position with thousands of tactical and positional ideas.
Gary Kasparov hints at such positions in his book 'How Life Imitates Chess' by saying something along the lines of taking a position, without care as to who is to move, and really concentrate on the essence of the position. The point being that it's not always important as to what the best move for White is or the best move for Black; the point is to recognize all the nasty little imbalances of every position. When you're done evaluating the position, calculate till you can't calculate anymore. Seems tough but I think I may give it a try as maybe a second Seven Circles.
You can find books with such positions online but since I'm such a nice guy, I'll post some links to some of the ones I think are the best. You be the judge of which to buy.
1) Chess Training Pocket Book: 300 Most Important Positions and Ideas - For beginners. This book gives solutions to the positions and tells which color is to move.
2) GM-RAM - Very advanced book. 250 positions with absolutely no solutions and they don't give the reader a hint as to who's to move. Perfect for any serious student of the '300 positions'.