How can you lose an autograph?

hi all!

I want to raise some money for Children In Need this year and have decided to do a sponsored bike ride and try to get some signed pics etc. apparently Children In Need is going to be around nov 16th in 2012, so i was wondering when i should start asking for autographs, cuz i don't want to get them all really early and loose them, but at the same time i don't want to have to sell them last minute, so when should i ask??? also what should i write in the letter cuz i haven't even asked for autographs for myself yet :S also any other ideas for things i could do to raise money????

Thanks