Nice to know: each row, eacht column and each ninesome adds to 45 (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9). Use this fact while looking for missing figures.

We can also use this property when a small 'protuberance' arises to such a set, as is the case at the blue arrow below.  

Add all fields in the red area. Without 'bump' this would result in 3*45 = 135. The surplus belongs at the cell indicated by the blue arrow. Just check for yourself...

In this fashion many 'bulges' are possible. Look for them and use them.

And of course: keep on applying the sudoku rules when you have found a part of the solution...