I talked about the importance of keeping compost aerated in my Beginners Guide to Composting. This is important because the bacteria that break down the waste need oxygen to survive.
One of the ways that you can aerate the contents of your compost bin and thereby speed up the whole process is to turn the contents regularly.
In your compost heap the central area (the hottest part) will always be the most composted with the cooler outer areas being less so.
As to when, you should turn your compost heap a few weeks after you have finished adding material to it. If you are really keen you can turn it on a weekly basis which will speed up the whole process considerably.
To turn the heap, remove the contents, mix it up and return to the bin.
You will now find that you have introduced lots of air and less well composted material will make its way into the centre of the heap.
Turning a compost heap will also disturb any unwanted rodents lurking in the heap and help to move them on to pastures new.