Someone once told me that once you decide to watch a movie you should no longer watch trailers for it. My experience has been that it was a good piece of advice. It's INSANE how much of the plot some trailers give away. I remember people put together the entire plot of Prometheus before it was released just from all the spoilers in the trailers, and that's not even the worst offender.
This is what I have always done. I have a very good memory, and I've lost count how many times I've been let down by a film purely because of the trailer(s) beforehand just exposing all elements.
I've been doing this with games and movies for years. It's so much fun to go into something blind with zero expectations, makes the whole experience a lot more exciting.
None of this "THE. TRAILER. STARTS. NOW" before the trailer starts
Trailers played in movie theaters do not include this. That is only used for online ads, because sometimes the trailer will play as a pre-roll ad and they don't want people to skip it.
The trailer shouldn't give the entire plot away
There's a reason studios do this. They run focus groups for trailers, and studieshave shown that the more plot they reveal, the more likely audiences will buy a ticket to see the movie.
I've actually seen trailers play in the theater that say that whole "TRAILER STARTS NOW" thing. It's like the studios didn't want to pay for another trailer so they just used the Internet version.
On the flip side, it big sucks when a trailer shows all of the cool parts, and it turns out those couple of moments were the only good parts of the movie.
The trailer is about ~2 minutes of different action scenes that looks fantastic. Then when you see the movie it's a snooze fest minus the few minutes of action, all of which were shown in the trailer.