We have been using a resource known at my school as the maths toolbox to show students strategies to deal with unknown and difficult math problems for a number of years. There are 12 in all and in the next few days I will be adding a presentation explaining how and when to use each of these.
I have just put together some posters for you to download in an A3 and A4 format that I would strongly recommend sticking in your classroom and your students workbooks to quickly refer to when unsure of how to attack a difficult problem.
These concepts can be pitched at any age and are a must have for all teachers to add to their own tool kit. I'd love to hear of any other great resources that you have to share also.