Get on www.fgw5pounds.co.uk
Free £5 voucher for any train ticket purchase over £6. You can register as many times as you want (and so get as many vouchers as you want), just need different email address each time. You then have to book your tickets through that link, but you can buy any tickets (Virgin, CrossCountry, etc) through that site.
I think you can only book until the end of this month, but at least you can get the discount for the first couple of away days.
Discount is £5 per booking too. So if you're buying two advance single tickets over £6, do it in two separate transactions. I got my Orient train tickets for £6 each way instead of £11 each way.
Cheers.