Well this one time I benched... oh, improve your chess. Nevermind.
PUSH IT TO THE LIMIT
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The hardest (but often most helpful, once you have acquired much knowledge about the game) thing that's necessary to improve your game is where you, essentially, go to a gym for your mind. Analyzing very tactical games, calculating hard puzzles, stuff like this makes your mind tired but helps a ton in that area. Then trying to defeat the computer in technical endgame positions, whether it's a basic mate (q + k vs k + r, b + n vs k), rook and pawn endgame up a pawn etc it just demands lots of practice and hard work, until they are burned into your mind and you can play them with your eyes closed. Those two things I have been doing and need to keep doing to take my game to the next level.
Sometimes really learning how to play your openings really well (and picking the right ones!) can take work too, though it depends on your strength how important it is. If you are good at other areas of the game, this extra knowledge in openings means that you're always in control when you're playing them and know exactly what to do. I have often outplayed my opponents in the opening, kept the pressure going in the middlegame, often won material, but many times failed to convert in an ending, though I have been getting better.
Hardest thing i have done to improve chess is that i have played chess 
What is the hardest thing you ever had to do to improve your chess and how did you do it. example's
1.get use to playing higher rated player's like 300 elo higher and i did this by having one coach me and playing him each time i got the chance ?
2.Gettiing use to really good defensive style of play and i did this by playing every engine/player i knew who was big on position and playing them every chance i got and learning new stuff about defense by playing them (I actually did this one for a while)
Come on we all could learn something from this topic please post.