Shakespeare might have written 'a rose by any other name would smell as sweet', but if he'd called Romeo and Juliet 'Fred and Elsie' would the effect have been the same?  What you call your characters is tremendously important, and can be particularly full of pitfalls for writers of speculative fiction.

Fans of the SF and fantasy genres are used to unusual names and can easily cope with the likes of Obi Wan Kenobi or Galadriel, but if you're sending your story to a competition where the judges are more used to mainstream fiction it might be a good idea to limit your invented names to two syllables.  Names that look difficult to read or pronounce can be off-putting, so think twice before using apostrophes or piling up the consonants.  Of course, there are names like that in the real world, but this means the writer might fall foul of unexpected connotations.  Start a name with 'N-consonant' or end it with 'tl' and you might spark unwanted and inaccurate associations with Africa or South America, for instance.

If you're finding it hard to come up with names, try changing a letter in an everyday name.  'Steven' could become 'Stenen'.  Adding or removing a letter can also work e.g. 'Streven' or 'Teven'.  Whatever name you eventually choose, remember that it might not be immediately obvious which gender the character is.  It can be really off-putting to read a page about a character you envisage is male only to discover when you turn the page that it's female, or vice versa.  Try asking a friend to read the names out loud to see if they hesitate, and ask them what sort of person the name conjures up.

No matter how careful you are you can still come a cropper with names.  I once had a fantasy story published in a mainstream magazine.  The editor didn't pay proper attention when she spell-checked it, with the result that the characters' names were changed halfway through!  Very puzzling and annoying for the readers, very frustrating and irritating for me, but she needn't worry - I'm not going to name names.