Grab 'em from the get-go. Make your first sentence, your first frame, your first scene an unforgettable experience! Hook the reader in and never let them go.
It's an important writing tip, but it's amazing how many comics don't follow it, even comics that are now very popular or recognized as classics. Garfield and Dilbert are entertaining enough, but Garfield's first strip is a lame "let's face the camera and introduce ourselves" routine, used by many other comic strip features and never successfully. Dilbert's is an odd Dr. Frankenstein routine that has Dilbert and Dogbert wildly out of character. Peanuts and Calvin and Hobbes, on the other hand, have the kind of first instalment that keeps people coming back for more. Some classic comic books aren't much better.
The first Archie story began by advising readers that Archie "hates the name Archie, so if you value life and limb, call him Chick!" Boy, that sure lasted.
Maus is an inarguable classic, but its primary hook is its cover. After picking up a haunting image of mouse-Jews and a cat's-head swastika, readers dive in to find... a quiet, slightly dull home visit between the author and his elderly, disagreeable father.
Some, like Chris Sims, theorize that the true beginning of a comic book is its first three pages, since the act of turning to the second page creates a two-page spread. But this is slowly becoming less true in the age of digital reading, and even if it weren't, the first page is still a distinctive experience. And in the hands of a skilled few, it can be a magical one.
T Campbell has written quite a few online comics series and selected work for Marvel, Archie and Tokyopop. His longest-running works are Fans, Penny and Aggie-- and his current project with co-writer Phil Kahn, Guilded Age.