Take this advice for what it's worth. I'm not at the very top or anything, but I've hit two plateaus that have eventually been exceeded, based in large part on the stuff outlined here.
The biggest thing is crutches. The main difference, I think, between level 30 and level 40+ is the extent to which players can build strategies based on the board, rather than based on which cards they like/feel comfortable with. My biggest jump (from mid 20s to upper 30s) perfectly coincided with a concentrated effort to play cards that I don't feel totally sure about. That's when I started to get a much better grasp on long-term deck construction - as I could no longer just drop strategies into prefigured molds.
I think there are basically three ways to lose games, once you've passed beyond basic strategic understanding. 1) pure bad luck, in a well-chosen strategy, 2) poor execution of a good strategy, 3) failing to comprehend what my deck is going to look like around turn 12 with the strategy you're pursuing.
Every player falls victim to #1. The very best players, it seems to me, don't fall victim to #2 very often, and have almost entirely eradicated #3.
I think #3 is really the thing to focus on improving. The easiest example is forgetting to buy money. Most of us go through that phase very early on. Another big thing is to build toward a realistic expectation of your draw, rather than a perfect one. That is, your combos need to have a reasonable likelihood of actually hitting. Beyond that, don't buy 'strong' cards just because they're there; make sure that they actually integrate into a strategy. Another thing is to anticipate their strategic choices and make sure your responses will integrate into the deck you're building. Will you need to purchase reaction cards, or cards that compensate for attacks they might play? Will you need to buy a few cards to prevent them from rushing the whole pile? Militia isn't a guaranteed buy, for example. But if they do make it a focal point of their strategy, are you going to feel obliged mid-game to start buying Menageries or Watchtowers to re-build your hands? And if so, will you have a deck that will make that workable?
In an ideal world, when the second re-shuffle hits you should already have a good sense (within a turn or two) of when you're going to buy your 1st and 4th provinces. I only manage that ideal very rarely, but my best managed games are the ones where that happens.
Final thought: playing the very top players is good, but I wouldn't agree with those who say it's absolutely essential. There are very good players even in the 10s and 20s. In fact, it can be useful to play them simply because they'll make the occasional mistake which will give you a chance to develop a better sense of the endgame. If you're consistently getting thwomped by the top 50 players you may be able to observe what they're doing, but that won't necessarily help you actually learn it.
I have found that I learn the most from playing people around the 20-30 range, and I get the best measurement of how I'm actually improving by playing the 35+s.
