One of the reasons (amongst many others) why my patzer rating is as it is today is because of my poor tactical vision and strategical skills. In other words, the ability to calculate accurately is still some thing I find extremely hard to get better at.
From looking at some of my games, I find that the majority of my losses are due to inaccuracies in my calculation of variations. You know you got your calculation very very wrong when Fritz evaluations changes by more than 1.0 points (eg. from a +0.6 toa -0.4).
Even more alarming is when the difference between your move and the computer's suggested move is different by more than 1.0 (eg. inability to increase from a +1.7 to a +2.9). While it may seem that the advantage is still there, failure to capitalise on minor mistakes made by your opponent means that my ability to increase my advantage is sorely lacking and below par.
So how does one improve on such areas?
Unfortunately, there is no shortcut to success. Constantly working on tactical puzzles and problems is still the key to evaluating a position accurately and finding the best move wherein possible. Learning and knowing how to increase your advantage is slightly more tricky. You need to be able to not only have a good grasp of tactics but also to be able to understand the nature of the position (eg. should I trade pieces, should I push that pawn). This comes with working on specific positional puzzles and problems and working through GM games.