go with something simple, and something that you think fits. i tend to mark mine as "autograph request" though i do of course use the word charity in there too to fit my request.

Hi Guys,

Just wondering on people's opinions on what sort of subject line work best for email requests? eg 'Autograph Request'/'Signed Photo Request'/'Fanmail' ect

I've only done a few in the past (much prefer handwriting letters) but have seen some really good successes from people I'm quite a fan of