Here are two similar strategies used for increasing speed of solving the puzzle:
1) Pick the number that has been solved for the most (or at least a lot) in the sudoku. For each 3x3 block that does not contain this number, look for 3x3 blocks in the same row and column as this 3x3 block that do contain this most solved for number and eliminate places where this most solved number can go in the 3x3 block that does not contain this most solved for number. You will find a solution when you limit the place this most solved for number can go down to one place. An example
The number '9' has been solved for six times in the puzzle, in all but 3 of the 3x3 blocks. In two out of three of these blocks, it is possible to solve for where the '9' goes by eliminating places it can go by looking at the location of '9's in 3x3 blocks in the corresponding row and column of this 3x3 block.
2) This is basically the opposite of 1. You start with rows, columns, or 3x3 blocks that are nearly solved, figure out which values are missing, then eliminate possibilities by looking at 3x3 blocks in the same row or column as the nearly solved 3x3 block that contains the missing values (and thus can eliminate possibilities in the nearly solved 3x3 block). An example
The top middle and bottom middle blocks of this sudoku are basically identical and nearly solved. In the top middle block the three unsolved values are 1, 4, and 9. Looking at the top left block's 4 location will eliminate the 4 being in the top left and top right cells of the top middle 3x3 block, so the 4 must be in the middle position of the top middle 3x3 block. The same is true with the 6 for the bottom middle block after examining the bottom right block's 6 position.