Usually have better luck at smaller cons, less 
people to compete with.  At Otakon, if they have 
a session on Friday or Sunday probably a much 
higher chance of getting something then - 
Saturday will be a zoo and probably not everyone 
will get an autograph.

I know I was very disappointed not to get Hikaru 
Midorikawa's autograph as Otakon just had one 
signing session in 2001.  I am hoping to get TM 
Revolution's autograph this year - but not 
holding my breath since it is looking like Otakon 
will possibly become US largest anime con this 
year over AX as Otakon already has over 9000 
people pre-regged