In business, when you want to sell a concept, you need a word or phrase to communicate your idea to potential buyers. Often you can cook up this brand name by raiding the description of your product, as Microsoft did (microcomputer software.)
Writers do basically the same thing when they create novel and series titles. We try to hit the perfect word or words that will capture the browsing reader's attention. Personally I prefer one-word titles like StarDoc and concepts like Darkyn, but I also like hitting the poetry books and listening to music to get ideas. If Angels Burn came from a rather terrifying poem by Pushkin. My JH novel Into the Fire was originally titled Dance Into the Fire, a line from the theme song of View to a Kill.
Darkyn is a splice of two words, dark and kyn, the latter being a medieval spelling of kin. Unlike StarDoc, which hit me out of the blue while I was taking a shower, I spent an entire week scribbling on a pad and trying different combinations of words before I hit on Darkyn as the brand for the series.
Coming up with a new, memorable catch phrase for a title or series concept is simply a matter of playing with words. Keeping it simple is really the trick. Writers love to be wordy, which is why many have titles that read like bad bumper stickers. If you want your concept to jump out at people, don't load it up with a lot of artistic baggage. Make a list of single words that fit your novel's time period, setting, protagonist, theme, conflict and plot, i.e.:
Once you have your basic list, take each word and play word association. You don't have to keep making lists, if you're not a list person, but do try combining and recombining different words. If you're stuck for words, head over to a site like Vocabulary Helper. My personal favorite is the Visual Thesaurus, because it presents the information the way writers and other creative people tend to think.
Remember to have fun with it, too. Make a game of it with friends. Approaching it with a sense of humor will relax you and often turn up ideas you might otherwise have been too uptight to recognize.