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Sometimes if it's a celeb that i really like or I find a picture that is really darling I'll send an 8x10.
I only send at most one 8x10 per request. (There was one exception.) The others I send 4x6. (I have sent some 5x7 but I don't find them really worth the extra cost to print or mail.)
I usually send no more than three pictures. I've sent up to four pictures though. I've told the celeb that I couldn't pick so now they have the choice of which, if any to sign. That really is kind of excessive though.
In the letter I ask that all photos be returned, even if they are unsigned.
I put a post it note on one of the pictures asking them to personalize to ___ (my name). I make it to where the sticky part is on the back of the photo and I fold it over to where the side I wrote on is in front of the photo... in order not to damage the photo.
I always send 9x12 SASEs, even if the pictures I'm sending are smaller. That way if the celeb wants to add his or her own picture they can, regardless of its size.
Before when I was not sending pictures I would use two stamps on the SASE. Now that I'm doing it this way I send the SASE with three stamps.
As for IRCs it's a really nice idea to send some with every overseas request. It's probably not enough to send just one IRC if it's a 9x12 envelope. I send two IRCs. In the past I've been cheap and sent none or one, but that really isn't considerate to the celeb. It makes me feel bad so I don't do it anymore.
This is just the way I do things. Many would find sending that number of pictures to not be a good idea.
With time and practice you'll find the ways that suit you best. Enjoy your new hobby. It can be very fun.