If you use an annual travelcard, then chances are you've got an Annual Gold Card too.
You'll probably know this scores you a third off rail travel in parts of the UK. You may also be aware that it gets you 2-for-1 discounts on attractions, including many in London.
But there's a third boon to the Gold Card, which you may not be taking advantage of — because you can also apply the Gold Card to your Oyster card, earning a 1/3 discount off all off-peak travel.
How does it work?
Let's say you've got a yearly zones 1-2 travelcard. All your travel within zones 1 and 2 is covered already. But if you travel into zone 3 and beyond, you have to top up your card with pay as you go credit. It's this extra credit which you can get 1/3 off. (Note: the discount only applies during off-peak times.)
How do I do it?
A slightly fiddly combination of online and in-person registration. As TfL's website explains, you'll need to register your Oyster card online. You'll also need to ask someone at a tube, London Overground or TfL Rail station to add your discount. (Staff at some National Rail ticket offices, Oyster ticket stops and TfL visitor centres can help you out too.) Show them your Gold Card and give them a password of your choice.
You're not quite done yet. Now you need to go back into your online Oyster account and type in the same password you've told the staff member. NOW the discount should be applied. Phew. (Remember to carry your Gold Card with you whenever you plan on using your Oyster discount, just in case.)
How much can I save?
That totally depends on how much you use the card. We reckon if you live/work in zone 1-2 (and have a travelcard to that effect) but are also the Londonist-digesting, adventurous type, you could very easily spend £100 on top-up credit each year. In which case, you'll make a saving of about £33. The craft beers are on you. But depending on how much you head into London's outer limits during the weekend, the saving could be significantly more. Especially if you have some weird obsession with the mysteries of zones 7-9.
And remember: you'll have to go through this rigmarole every time you get a new annual travelcard. So by our reckoning, that's once a year.