I tend to go across the boxes first of all, top three, middle three then bottom three. Check out 1s, then 2s and so on. Having done the horizontals, I then take the first three boxes going vertically, then the middle three and then the three on the right. After that I'll look at individual boxes that have a lot of number filled in and see if logically I can get some more. Or if there is any line that has a lot of numbers, try and complete it. Then sometimes you can go across a line trying to fit a number in and realize, it can't go there, can't go there and so on until there is only one place it can go.
Hope this helps.