I look for exclusion sets. What I mean is, there are a number of boxes that MUST contain numbers from the same subset. Simplest is a pair, but 3s can be very handy as well.
This example isn't from your puzzle, but lets say you've got a row that looks like this (big numbers are givens/filled in for sure, slashes are the only options for those boxes, based on unseen clues)
|2 __ __ | 3/6 __ 6/8 | __ 7 3/6/8|
Those blank spaces cannot contain a 3,6, or 8 since you know they have to go into 3 other spots that row. So those blanks must therefore contain the numbers 1,4,5, and 9.
One other technique that I use when stuck, though more for Kakuro than Sudoko, is to take a space with 2 or 3 options, pick one, and follow the chain of possibilities (in my head or on some scrap paper or something) that the choice forces until one of 3 things happen:
- results in a contradiction
- it solves the puzzle, or
- things stall out inconclusively one way or the other
If #3 happens, I just choose another option for the same box and go through it again. If thats inconclusive too (very rare), I'll go to another box not directly linked to the first one.
This shouldn't been needed for Sudoku (at least, not anymore ... there was a time where it was required on some of the most difficult puzzles), but its real handy when you've got a half solved puzzle and don't see a way forward.