Depending on which con you go to and depending on 
the artist, sketches can be easy/impossible.

Reguardless, you'll probably have to wait in a 
VERY long line.

Anime Expo is a PAIN to get sketches at.  They 
have a lotto system that makes it very difficult
for any fan who wants to get a sketch to get one.

At Otakon or Anime Central, pretty much anyone 
willing to wait in the insanely long lines is 
very likely to get one.

However, if you go to a rather small con that 
somehow got your favorite artist, you're most 
likely to get a sketch there.

Having friends in the anime industry can really 
make things easier too. Some artists may go to
cons un-announced, and if you catch them and ask
really nicely, you might be able to get a sketch.

There are also signing events in Japan. The lines 
are killer and you'll have to start waiting way 
early but it's well worth it!