We could really do with a gender neutral singular personal pronoun ("it" never seems appropriate as a personal pronoun) but sadly we simply don't have one. There's no single correct answer for how to deal with this, as much of it is embroiled in personal preferences. Common approaches include:
- Using "he or she" or "she or he" throughout. In writing this is often shortened to "he/she", "s/he" or "(s)he".
- Choosing one of "he" or "she" (more or less at random) and sticking to it - possibly alternating between them if using lots of different examples. Sometimes this approach is accompanied by an explanatory footnote
- Using "they" instead of a singular pronoun. Looking up the definition of "they", "used to refer to a person of unspecified sex" is certainly one of the options listed; it is not incorrect to use "they" to refer to a single individual.
My personal preference is the latter, as it strikes me as less cumbersome and retains consistency between the written and spoken word. In my experience this seems to be becoming the more popular approach, although I haven't conducted any research on the matter. As I said earlier though, there is no single objectively correct or most preferable approach.
Edit: The Urban Dictionary lists "thon" as a gender neutral singular personal pronoun but the more conventional dictionaries I looked at don't list that definition, and it has yet to become established in common usage.