I used to do this regularly but even though I receive many seemingly 'unfranked' stamps on post at Christmas I no longer bother reusing them. My worry is that the stamp may in fact have been franked but the mark is not visible to me so the Royal Mail may reject it if I try to reuse it. I send very few things by post now but everything I do send is important - and I don't see the point in taking a risk that an item doesn't get delivered. Maybe I'm missing out on around £5 of savings each year, especially as stamps are going up again by 3p or 5p shortly