Many writers are uncomfortable about starting a book without having found the right name for at least their main character. It’s easier to work with a "working title" for the book than a "working name" for a character. Once the author gets to know their characters, their names can become almost as entrenched as the writer’s own, and having to change them can be very off putting.
It's worth keeping in mind, though, that any situation could force a change – from making new friends with the same name as your character, to new celebrities springing up to claim your protagonist's name and slap a stereotype on it forever. A few years ago, Paris was a boy's name, but using it for a male character now could confuse your readers completely.
Bear in mind, also, that some popular names date very quickly and the book writing and publishing business often takes a number of years. You could try to predict when your book will be on the shelves, add to that the age of your character, and work backwards to find a suitable name.
Popular with fantasy writers in particular is using exotic-sounding made-up names – the more unpronounceable, it seems sometimes, the better. However, this practice inevitably creates a distance between your work and your readers. It's difficult enough to convince readers to pick up your book. When a reader is confronted with a bunch of consonants and apostrophes the first problem they encounter is imagining what gender this character is, and, depending on the genre, whether the character is even human.
A writer runs a real risk by giving a character an opposite gender name, too. Women with men's names are more common than the other way around. Try to make it as easy as possible for your readers to understand your writing.
Ambiguity makes it too easy for the wrong assumption to jar your reader, and that weakens the magical hold your story has over them. If you must use something unusual, try to ensure you describe the character immediately so that the reader can reconcile and imagine the character. Try not to block a reader's ability to visualise or your character will remain flat and two dimensional to them. When the reader is unable to identify with your characters, they will not care what happens to them, and might close the book and pick up something they can identify with.
Finally, be aware of the pronunciation of your characters' names. It can be frustrating for a reader to have to try to wrap their tongue around a name that is unfamiliar. It's quite simple to email a group of your friends and ask them if they find your proposed name easy to read or if they trip over it and end up making up their own pronunciation. If you get too many negative responses, consider a different name, for the sake of your future readers.
Elsa Neal runs a Peer Critique Service at HearWriteNow.com.