It is important to remain current within your own specialist subject area and in particular with current specifications that awarding bodies prescribe,. In this regard it might be useful to do some research into the modern course content of your subject.
For example if you teach maths, it might be helpful to look at the current GCSE syllabus and some recent GCSE papers to get a feel for what is being delivered in the classroom.
You could look at some college prospectuses [they are usually on line] to see what qualifications are being offered in your area and then research these to see how things have changed since you last taught them; perhaps even think about what kind of scheme of work you might devise to deliver them. This exercise might also indicate areas where a bit of updating would be appropriate.
The following is a list of topics we have compiled some notes on, each linking to another page