I don't really believe that a petition has an effect. Fan petitions are the bare minimum effort for fans online, and everyone knows it. I think the only thing that has effect when you DON'T have the option to vote with your dollars (such in the case where there character is not appearing regularly and the few places she does appear aren't advertised--and any loss of revenue to the current Batgirl probably wouldn't be attributed to the change without massive feedback to that effect) is the mail.
To this end, DC has given us an opportunity by opening the letter columns:
To submit a letter go to:Imagine how strong a message it would be if next month's columns were filled with requests to see Cass Cain?
Letters may be sent by regular mail to this address:
Letters to the Editor
New York, NY 10019
Please include your full name, and address, for confirmation purposes. Letters should be no longer than 500 words and should not include attachments.
Letters may be edited for length or clarity and may be published in any medium. Letters become the property of DC Comics.
Unpublished letters will not be returned or acknowledged. Published letters may identify the writer by first name, hometown, state or country.
Now I don't have the time, energy or money to organize a planned letter campaign like Girl-wonder.org did, but until we find someone who does the best thing to do is to flood them with Cass Cain letters. Come on, take a few minutes and write out why you like Cass Cain and would read a book starring her again. Send an email, send a postcard, send a short note, send a long note. Then read the lettercolumns next month... and answer back if the answer isn't "You will see her here."
Now, I wasn't going to mention it because I figured I implied we'd want to send publishable letters in, but there is a certain decorum needed. I understand not wanting to but we should keep it positive, y'know?
As annoying as it is, it's also worth noting that Gail Simone recently said that tone is important when sending letters about Cass.So stick with what you like about her and, more importantly, that you would pay to see her back.
I know we have every right to be snarky and rude at this point, but she said sending a bunch of "FUCK YOU. BRING BACK CASS!" messages is only going to make editors think that they'll get screamed at no matter what, and make it less likely for her to ever come out of limbo.
Especially write in if you're not reading Batgirl, and let them know you're not really feeling this other book but you'd try one with Cass Cain.