The easiest way to achieve this is to keep on completing the same puzzle fast over and over again.
The 9x9 puzzles are probably best because they net you about 90,000-95,000 dependng on you time and if you make any mistakes.
Also the website below solves 9x9 puzzles for you, so just pick one of the 9x9s, use the solver and keep on doing that puzzle as fast as you can.
http://www.sudoku-solutions.com/sudokusolver.php