When I get stuck I try these:
1. Like Heureusement, take a break for a few minutes - sometimes you spot something obvious when you go back to it.
2. If it's a paper puzzle, dub it into computer program (I use Pappocom and Sadman - more usually the latter these days because of auto candidate entry which can be useful) - sometimes you spot something when it's enlarged on screen or, with Sadman and some other software, you can get the next step and the reasons for it through solver log.
3. Post the puzzle to the forum and ask for help from the other good folks who subscribe. Somebody is sure to be able to give you a clue to the next step.