Sign up your characters for cameos!
At some point during his or hers comic, the artists sometimes decides to bring a host of cameos into a storlyine for some reason or another, to be used as background chracters, temporary castmembers etc.

Usually the artist in speaking decides on showing off a group of characters from comics they want to pay homage to, but... other decide to send out an official call of "Hey! Sign up your characters and I'll include them in a upcoming storyline!" which opens up for a whole line of interesting opportunities... and challenges!

As compared to a cameoparty where the artist makes all the decisions regarding the cast, inviting people to submitting their own characters usually helps to generate even more focus on the webcomic and evnet in speaking as the words will spread out, and artists might end up showing up from afar to sign up their characters and keep an eye on the comic as long as the storyline runs, sometimes staying and keeping up even after the event has run its course...
However, this method also challenges the artists' skills to a greater degree as he/she cannot choose what characters to present, as fitting to their style, and are forces to honor their promise and show who signs up, no matter who sign up!
... off course, that can also be half the fun for the artist! :)

These days at least two comics are examples of the latter approach to cameo-appearances...

Artist Ron of Magical Misfits is these days holding a Shower Party for his character Dark Eyes who got pregnant with Brian from the comic of Drunken Scribblings during one of the many crossover events the two of them have taken part in during the last couple of years.
With such a history, what would be more natural than inviting a host of women from other comics, both those he wanted to show homage to, but also everyone else who wanted to show up? :D
From good sources I know enough to tell that the amount of feedback on the invitation totally surprised Ron, and he will, gratefully, have to work overtime on getting all featured in the Shower Party Storyline! Nonetheless, the invitation is still up, and in this moment of writing its still possible to sign up your chracters in MM's forum if you would like to see them featured.
The storyline starts here, and the cameos are arriving, bearing presents, as we speak :)

The other comic in speaking is David A. Davis' Cosmic Dash where David has hinted of an upcoming event of the Cosmic Dash Continuous Crossover Continuum in which he plans of showing off submitted characters in various roles of cameos, depending on how they fit in.... all characters will off course be adapted in art to the style of Cosmic Dash!
Not having started up yet, there's still time to sign up on this page if you want to take this unique opportunity to see your characters in another comic and another style! :)

Remember... you are not whoring out your comic of characters if you sign them up for something like this. If people ask its because they want them, and is there a better way than giving them what they ask for?