Step 1: Don't Say Anything
It might sound harsh, but the only way to truly avoid spoiling something is to keep your mouth shut. When you're around someone who hasn't seen or read up to the same point as you, just don't say anything at all.
Talking in code or leaving out characters' names doesn't work (it's not nearly as cryptic as you might think): "Could you believe when you-know-who got trampled by that llama?" Now, the next time that person sees a llama on the show, all they're thinking is: OK, who's this camelid gonna flatten? Of course, it may turn out that it's actually an alpaca and the llama-trampling doesn't happen until season 4 (Attack of the Llamas!), but the fact that they already have preconceived notions about what may or may not happen has changed the viewing experience.
Even a comment as innocent-sounding as "Man, Billy Bob is my favoritest character ever!" is spoiling something. If the person talking is on book 5 of the series, and you're only on book 3, you've just had several things ruined. Number one, you know that Billy Bob is still alive in book 5. When you get to the ninja attack at the end of book 3, you know that he will survive. Same with the nuclear blast in book 4. You can also ascertain that Billy Bob hasn't done anything particularly despicable (like betray Princess Cucumber), else he probably wouldn't be someone's favorite character (there could be some debate on this point, though).
Posting something on social media like "Lizard Wars Season 3 finale... sad face" may seem harmless. But every person who reads this who hasn't yet seen the finale just learned that something bad happens. Part-way through the episode they may even be able to guess what that is, before it occurs. Social media is the worst for spoilers, because it is very easy to stumble upon something unintentionally, and can ruin things for multiple people.
I know it's tempting to vent or to give your two cents after a particularly stirring or controversial episode - I've been there, too. But please, try to refrain. In person, talk privately with those who are at the same place as you. Online, use extra caution, and when in doubt, don't post at all. It's that simple. If you care about not spoiling things for others, you will follow this one easy rule: don't say anything.
