If you're having trouble on a level, play an EVEN HARDER level for a bit, which'll make the other level seem easier in comparison.
+1
I don't know the neurological processes behind this, but my theory is that your brain tries to adjust your perception of time to the game. When I see a video of Hexagonest, it seems quicker than it seems when I actually play it. When you play on a harder level, your brain adjusts your perception of time to that level; and then when you switch back to the slower level, you'll do better, because your brain works quicker since it's still being conditioned to the faster level. This is only my layman's theory, so probably it's just bullshit.
There have been studies that prove that the brain "slows down time" during a crisis, to give you more time to react. I suppose some of the waves in this game *would* count as a crisis 
On a further note, while playing
Hexagonest helped me to get conditioned for
Hexagon and
Hexagoner; nothing seems to help me against
Hexagonest itself. Playing
Hyper Hexagon and
Hyper Hexagoner is good; and actually I can condition myself to get better scores on them than on
Hexagonest, but they don't actually help me to get better scores on
Hexagonest. For some reason, on
Hexagonest I usually die around ~17 +/- 3 seconds over and over again within an astonishing number of tries. Today I was playing the game for hours, still I don't seem to get better.

Although I have a 40 seconds record, it was really hard to achieve - sometimes I get a feel for it, and I do 2-3 relatively good runs; but then I revert back to the ~17 seconds average.
Hexagonest might have another secret... Yes, despite my brain tries to adjust to the speed, it just doesn't seem to be enough; still I don't think the core difficulty of the level is dealing with the speed...
Hyper Hexagon seems faster, actually, still I'm able to get better scores on that - heck, today I've
almost passed Hyper Hexagon (got 54 seconds)
well before I would be able to pass
Hexagonest. I think the main difficulty of
Hexagonest is in the patterns: and not in the individual patterns; rather in the uncanny sequencing of those patterns. You concentrate on a pattern and then no matter how easy one comes after; you're just not prepared for that. You know how many times I died immediately
before and
after spirals those only require you to mindlessly press the button for a few seconds?...
Actually, one thing is sure,
Super Hexagon does good to the nervous system - I'm known to be a slow guy, and I didn't even believe that I'm able to do what this game requires - passing the first 2 levels is already a big achievement for me... Maybe I'm not that bad, after all?

But still, I WANT to pass all 6 levels, and I MUST see the ending!