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Always have a plan
Always have a plan in your mind, without a plan you will be moving your pieces for no purpose at all. What exact board position is that you want to get? Only after proposing a plan you can come up with the necessary tactics to execute your plan.
Think in terms of squares instead of pieces
This is something which can completely change your game. Beginners mostly think of capturing pieces, and material advantage is all what they try to get. It can win you some games, but to move to the next level you have to think in terms of squares. Which squares are controlled by you? Which ones are controlled by your opponent? Which ones should you control?
When attacking the king, you must observe all the squares that are open for the king to move to. Your job is to close all those squares. This job is made easy once you start thinking in terms of squares.
Evaluation of opponent's moves
Keep a close eye on each opponent's move. Try to find the motives of his moves, in this way you can know of a threat in advance. As you keep track of the opponent's moves and the motives behind it, you cannot be surprised by your opponent.
Play against a smarter opponent
There is a saying:
“You can only get smarter by playing a smarter opponent”
Playing against a smarter opponent can improve your game incredibly, but not just the mere act of playing against that opponent would do it. While playing against the smarter opponent you must observe and evaluate his moves. This alone, would give you all the necessary tips you need to move to the next level. The smarter opponent can be a high level chess engine or any human opponent having a good ratings.
The basic tactics
Familiarize yourself with basic tactics like forks, skewers, pins etc.
In above example the Black Knight is pinned to the black queen and Black rook to the black king by two White bishops(shown by arrows).
Above is a simple example of skewer, check is delivered by White queen and Black king has to move out of check as it does. Black will lose his rook to the White queen.
Above is the classic knight fork, the black king has to move out of check and as it does, Black will lose his Rook to the White knight.
Watch out for the discovered checks. Beginners often miss to notice a discovered check which could be devastating and on most occasions cause heavy material loss. So you should use it yourself and should watch out for it to be used against you. Reuben Fine one of the great grandmasters of past has also pointed to this fact by saying:
"Discovered check is the dive bomber of the chess board"
Above is a simple example of the discovered check. As the White knight moves, this exposes the black king to the white bishop thus giving a check thorough the bishop and at the same time attacking the black queen. So the black king has to move out of check as there is no other legal move. Thus Black cannot protect his queen which would be captured by the White Knight in next move, as you can see the discovered check can be really devastating.
Practice the basic moves of pieces after removing all other pieces from the board. Especially knight moves, as the knight moves are one of the most difficult for beginners to grab. A knight can create great opportunities and some deadly forks when you know perfectly well about its movements. Same goes for the other pieces.
Learn to move the Knight to almost every position on empty board.
Practice different positions by placing different pieces in different ways. Practice the positions about which you have some confusion. Explore all the possible moves in that position. You can also practice in this way online with PGN editor.
Never give-up mentally
As a beginner or intermediate player it is possible that you will lose hope if you are unable to execute your plan or lose a key piece, but losing hope is actually a bigger loss that losing a piece, because when you lose hope you are most likely going to lose the game. In a game of chess anything can happen and it's not over till the last movement, even if your opponent has significant material advantage over you and has a better positioned pieces also. There is always a chance that he will commit a mistake, even if he doesn’t, there might still be an opportunity as the game progresses. But you cannot avail that opportunity when you give up mentally.