Here is an exercise, promoted by Jeremy Silman in "How to Reassess Your Chess - The Complete Chess Mastery Course", to practice and improve your calculation skills. A Word of Warning: this exercise is difficult and time-consuming! However, I think it is one of the most useful and improving exercises you can do.
Take an annotated game from one of your favorite players and play out the first 10 moves or so until you reach the beginnings of the middlegame, stopping when it is your move (your favorite players move, that is).
Now, cover up the moves and figure out what is going on, using your thinking technique (or Silmans', if you prefer, by recording the imbalances, etc.) and derive your candidate moves. Write down all your candidate moves! Now, without moving the pieces, analyze out each candidate move in your head. Analyze each branch as far as you think you need to.
After you are done with this move, make the move in the game and the response and do it all over again. Continue to do this until you complete the game.
Afterwards, go over the game annotations and compare them to your annotations, paying attention to the logic and the tactics in the game and how close you came to the actual play. Play over the correct analysis when it presents itself different from your own.
This method should help you in developing your calculation and tactical skills and plan development.
Enjoy and Merry Christ*mas to everyone, Christian or otherwise!