Heh, first time I met John was SDCC '99 and he was wandering around with a large art folio containing 4 Drew Struzan pieces including an awesome Indiana Jones and a Star Wars piece. My eyes bugged out when I saw the Struzans!
My eyes bugged out again later in the evening at drinks when Jim Reid brought out a folio of his latest acquisitions including an awesome Frazetta piece *sigh* I just looked on in awe.
As for tips, I don't have anything original to add other than (if you can't contact an artist before the con) ensuring you know where the artists are located.
Just because they're not in Artist Alley doesn't mean they're not attached to an exhibitor booth. A careful perusal of the program to check out the exhibitor booth listings and Artist Alley is worthwhile.
As an example, Dave Gibbons will be attending this year but not attached to an obvious (ie comics related) booth. Thankfully I've already made arrangements with most of the artists I want to catch up with so I don't have too many to ferret out