I think it was on Sydenham Forum that I found a useful piece of advice:

If you have a gold card (annual season ticket) you are entitled to a discount on PAYG Oyster (and other rail fares). You need to take your gold card and Oyster PAYG to an underground station (not Overground) and they can make the Oyster change in a couple of minutes.

This gives you a 35% discount on Oyster PAYG outside of peak times.