Jim certainly gives good advice, and I agree...his bit on LJ about character development was a huge help to me. In my reading and library work, I've run across some memorable characters: Harry Dresden, obviously; of course Bob is as well, because now everyone wants a talking skull...
Other characters like Douglas Preston and Lincoln Child's Special Agent Aloysius Pendergast, Kim Harrison's Rachel Morgan, Janet Evanovich's Stephanie Plum, and Jeffrey Deaver's Lincoln Rhyme are all very memorable characters in modern fiction.
They're all memorable for different reasons. Harry is sarcastic and has a great sense of humor, but what makes him stick out, for me, is his heart. Yes, he may sling spells and have more than a few questionable allies in his line of work, but he is a guy who will do whatever it takes to protect the ones he cares about. What also strikes me with Harry is that his character has evolved, but not disappeared. There are other writers, whom I won't mention, whose characters have lost their touch, their uniqueness.
The other characters I mentioned have very unique attributes to them as well. Special Agent Pendergast is a New Orleans native, old money, whose complexion is white as a sheet, and he wears only black, tailored suits. He's a brilliant mind who has old-world tastes and a general...oddness about him. Of course, having a psychotic younger brother out to kill you (Diogenes Pendergast) also helps.
Rachel Morgan's got her own array of spells and magic, along with a pixie sidekick, Stephanie Plum's a klutzy bounty hunter, and Lincoln Rhyme is a crippled, but brilliant, criminal analyst.
It's just like Jim said on his LJ (as I loop back around to the starting point of this reply)...you've got to make them memorable, because otherwise, they're as flat as the page they've been printed on.
