I would recommend starting the book with a "cast of characters" list – names and a very brief description. I have heard a number of readers say they wish every book had this. It can help new readers catch up, act as a reminder to old readers, and work as a reference if people get confused.

Most other suggestions are hard to implement if you already have one or more books in the series published, but in general, you want to make sure that names are quite different – vary the number of syllables, don't start two names with the same letter, etc. I personally have trouble with gender-neutral names, though I like them in real life. I sometimes read reviews or descriptions for books where the characters are named things like Sky and Tam, and I can't keep track which is male and which is female. In a world where names are not familiar to the reader, you might want to have a construction for male versus female.

You can also help differentiate characters through their mannerisms, though it's easy to go overboard with this, if you have one character constantly clearing his throat before speaking, or flipping his hair. But if you do this the first couple of times someone appears or speaks, it could help establish them in the reader's mind.