Did you know you can send your used postage stamps to the PSPA and they can make money from them? Trim stamps so no more than 1cm of envelope around them, don't cut too close as you don't want to damage the perforations, pop in an envelope & send to:
PSPA
Margaret Powell House
415a Midsummer Boulevard
Milton Keynes
MK19 3BN
I'm going to ask my friends to start saving theirs for me too.