There is always some contention surrounding theme selection, and I figured I would chip in my two cents as someone who's been through this a few times.

• Vote "yes" generously. I saw a lot of people last time around proudly submitting screenshots with all but 50 or so themes slaughtered. Don't do that! The theme slaughter is basically an approval vote with multiple winners. If too many people vote for only their favorite candidates, instead of voting for every candidate they would be okay with, it reduces the usefulness of the vote. If you want to be picky, that's what the final voting stage is for.
• Don't reject a theme just because it's a duplicate of one you already voted on. If you voted yes on a theme before, and you see it again, then vote yes again. Once the theme slaughter is over, someone will go through the top themes and discard similar ones before putting them up for a vote. So, don’t worry about duplicates, just try to vote consistently.
• Regarding interpretation of themes: This one is more subjective, but in my opinion, the best themes are ones that people will naturally interpret differently without trying, because of their unique personality and cultural perspective. Words and phrases with multiple meanings, or objects and concepts with many different applications, are good building blocks for such a theme. Descriptions of specific game mechanics or art styles, cultural references that not everyone will be familiar with, and ideas that only apply to a specific game genre (like platformer or FPS), generally don't work well as themes.