For my trilogy, I chose the series title "Broken Cosmos" because I wanted sound epic space saga but also rather dystopian and cyberpunk. It took a lot of thinking to work that out.
For the books in the trilogy: Florida Station, Martian Flight, Neptune's War, I focussed on main parts of the book. In book 1, the space station "Florida Station" is very important and the story starts there. In book 2, "Martian Flight", the characters are escaping to Mars. In book 3, "Neptune's War" a fleet has mobilised from Neptune to fight the invaders into the outer territories from the inner planets.
I can't say it was easy to come up with those titles. It took ages. But if you focus on pivotal moments, scenes, items, locations, while keeping the length of each title roughly equal across all the novels, it should be a good title.
One thing that pissed me off so much was that I had decided on Neptune's War and told my cover artist about it months before publication. I had checked on Amazon that there were no duplicates of that title, and I finally published only to find that the same title of a book (totally different series though) was published ONE DAY before I published. And I could not change the title of mine because that would have required redoing a whole lot of work with ISBNs, covers, reconverting, promotion and all that. And Neptune's War was a perfect title for my work. Oh well, I guess there are duplicate titles out there.