Just a mild-mannered girl trying to find the time to read her very large pile of books.
1. Aliens are not going to look like us. They're just not. Aliens as the new YA romance creature isn't a great idea, it's silly, because they're not going to look like us.
2. Technology. This is especially important if you're writing near future sci fi. You're going to look like a fool if you're writing a book set 50 years in the future and people don't yet have technology that exists or is under development right now, so be aware of what modern technology is and where it's going.
3. Research. A lot of it. Don't take the word of a single source on anything. Find more than one expert. Find more than one book. Find more than one article. Make sure everything you use is well-sourced, be careful how much you trust Wikipedia, and do not Dan Brown everything you learn.
Extra Tip: If you want your sci fi to be accessible to people other than hardcore sci fi nerds, take all that work you did and build it into the foundation, but don't infodump science details that mean nothing to the plot. It's science fiction, not a science text book.
Extra Extra Tip: Making everything into a homogeneous Western society is not the same thing as making it post-racial. You're not progressive simply because you named your character Jenny Li. Okay? Okay.