Study and practicing openings. By this I mean you should memorise all the lines of an opening; not just the main lines. A vast majority of matches are won/lost within 30-35 moves by a few botched up opening moves.

I don't recommend puzzles that much. They give you mental satisfaction. However, it is one thing to think of a good move knowing before hand that there's such a move possible but entirely different to recognise that you are in such a situation in the middle of a game.