I've got Network Railcard, which gives discounts subject to various conditions, such as weekend or fares greater than £13 (I think). Thus for getting back from Sunny Whitstable after a FNRTTC it works well, but doesn't for getting me to the start from Three Bridges on a Friday night, as the standard fare is less than £13*. The Network Railcard covers the old Southern Region, I think. From the National Rail website: "The Network Railcard spans 16 counties in the South East including London.You, and up to 3 friends and 4 children, can travel together and save 1/3 off rail fares." Cost is £30 a year.
The other Top Tip for getting back from a FNRTTC is to make friends and get a Group Save. According to the National Rail website "GroupSave offers groups of three to nine adults travelling together 1/3 off, travelling off-peak on participating train company services. " You need to be traveling to the same destination, which can work well if you're all traveling o0n from, say London, having returned from the sea side.
Advanced fares are another good way of getting cheaper fares. They're released, in limited numbers, 12 weeks before they can be used. You'll be tied to a specific train though.
If you're an old bugger a person with a wealth of experience, there's the Over 60 railcard. Discounts seem to be more wide ranging than the Network Railcard and the price is the same.
* For FNRTTC starting in That London I now ride to Gatwick an tap in using my contactless bank card, which works as an Oyster card. Oyster fares are generally less than Network Rail standard fares and the Oyster Zone extends down the Brighton line to Gatwick.