If you REALLY REALLY don't believe in allowing Trump to take office, don't just sign that little petition on Change.org. I can tell it has good intentions and from what I've read, even Trump agreed that the Electoral College is a problematic and imperfect system. But I read the letter that was going to be sent to the Electoral College and I could see that it wasn't enough. The petition itself isn't convincing enough either.
Find another way to reach the Electoral College, specifically the Electors chosen by your state, and include all of the recent developments thus far (why you think Pence as leader of the transition team is a bad idea, why you believe having a GOP controlled government will have a negative impact on the country, how Trump's election has led to an increase in hate crimes and enabled bigotry, etc.). Don't just say "Trump is gonna take away same-sex marriage!!" or "Trump is a racist xenophobe!!" Because people have been saying those things for months and it didn't matter. Talk about the economy. Talk about the environment. Talk about the dangers of letting someone who can't control his words online, speak to and about other world leaders. A really big number on a single online petition might sound effective and hopeful, but what would even be more effective is a really big number of actual letters and phone calls and e-mails.
Also be open to the idea that, even if you don't have the person you want in office, that doesn't mean it's hopeless.
I read an article written following the election on reaching out to Congress. I'll be brief about my criticism of social media and media in general--it's filled with lies. Everywhere. People are so quick to believe everything they read without looking at the shady URL underneath or researching the background of that "news" source. We became way too engaged in Twitter wars and hashtags and the comments section of every think-piece that we failed to realize that none of that mattered. Your fellow citizen that you disagree with isn't going to magically change anything. Look to the people who are in charge: Congress.
The article mentions an e-mail or a snail mail letter, but before you send either of those, PLEASE consider the phone call first. Talking to another human being is more effective than a bunch of words that may just be glazed over and then set aside or deleted or thrown away.
Don't waste your time tweeting at Congress members--or do, but also find other ways to reach out to them. Many of these people don't have time to manage their own social media accounts and have people do it for them, so they don't hear anything and it just gets absorbed by their staffers instead.
For years many of us have been led to believe that the Internet and social media will do everything for us because of the feel-good stories about hashtags and remarks about how many views or retweets or Likes something has. The harsh reality is that the power of social media has been abused and overused. Everyone loves a 90s throwback these days, how about a 90s throwback to how we used to communicate?