I have quite successfully aged photos in Photoshop, however, one thing that I can't manage to do successfully is add creases or non-straight folds. Any words of wisdom??

Create a separate background with creases in a new document then save it as a Displacement Map which can be applied using Filter > Distort > Displacement Map in Photoshop.
There are a zillion tutorials on displacement maps you can google.
Regards, Murray

Let me go into tutorials..thanks for the information...