Secret!
What is the secrets of strong chess player?

secret of strong chess players? Make every piece of yours difficult to capture. Every move should be at least a three move deep plan each time you launch an attack from the weakest area of the opponent's defense. My opinion. More importantly, concentrate! Keep your mind in a game when you are playing a game. Dont drift!
Memorization and repetition.
If you're playing against someone rated over 2000, they have played every position before that you will see in your game, they need only remember the correct moves. Calculation is useful in preparation, in competition, your goal is to -- beforehand -- load all the calculations into memory so you just have to recognize patterns OTB and remember the correct move.
Anyone who tells you otherwise is lying to protect their ego, as it does not feel nice when someone tells you you are 'just' memorizing.
Memorization and repetition.
If you're playing against someone rated over 2000, they have played every position before that you will see in your game, they need only remember the correct moves. Calculation is useful in preparation, in competition, your goal is to -- beforehand -- load all the calculations into memory so you just have to recognize patterns OTB and remember the correct move.
Anyone who tells you otherwise is lying to protect their ego, as it does not feel nice when someone tells you you are 'just' memorizing.
I tend to disagree, the statement "they have played every position before" cannot really be true, since - as you surely know - chess has such a vast amount of possible positions. Yes, memorization is indeed a part of the game, but this alone will not win a game.
I know a player who is very good in knowing theoretical lines, but when he has to play on his own (let's say after 15 or 20 moves), he has some trouble in finding the correct plan or spotting the right squares where to put his pieces further.
Calculation is important, but this is not only memorization of patterns. You should not forget about strategy, it is important to see the important squares, and to develop the correct plan according to them.
It is a common misconception that all IMs tend to complicate the game and try to out-calculate the opponent in wild positions. I lost against two IMs in quiet, dry positions, where there was not much to memorize, not much to calculate, just moving of pieces til the endgame.
I played draw against two IMs, who were (on the other hand) inclined for a tactical fight. Both made some miscalculations, one of them made an incorrect rook sacrifice. I was better in both games, but due to the rating gap, i was content with a draw.
The endgame is an important factor too (true, also here there is some kind of "memorization", but this is only part of it).
A good player does indeed understand the position, even when he has not seen it before. I'm not a really good player, i am quite happy with several draws against titled players (including a GM), but since i have played against several good players, i can tell that is was not better memorization which was the reason for the rating gap.
Ok, i have some wins against the rating area 1800 - 2100, i guess this is due to a mix out of: strategy, memorization (ok, i do admit that i sometimes know the lines better than my opponent) and tactics. I am not very good in the endgame, however i had also a win against a 2000+ player in the endgame, maybe an exception.
Repetition: ok, it is also important, but also not the only thing to be considered. Why do so many young (or very young) GMs exist? This cannot be explained by pure repetition.
So my propsals are:
1) strategy (even more important than tactics imo)
2) tactics
3) memorization (less important than commonly thought)
4) endgame
5) talent

^
Yes but even strategies are concepts that have been learnt and memorized at some point. And of course tactics, that are all about pattern memorization.
It's just that I memorize themes instead of exact moves like in the openings.
Take the smother mate for instance: it sticks in your memory so much that is very easy to remember once you've seen it, but I believe everybody had to see it somewhere first and had to say "whaaat?" the first time they saw it.

A lot of tedious work. Well, for most it's tedious anyway.
Review your games, review your openings, solve tactics, review those tactics. Play over GM games. Review your/annotator's analysis of those GM games. Read instructional books. Review your notes / important positions in those books.
Not 1 time, not 2 times. The number doesn't matter. Review until it's learned.
Do one or more of these for a few hours a day, every day for a few years, and you'll get better.
How to reach from 1800 to 2100?