I always follow the old rule of using the neuter "he", but a lot of people do not like this old rule. I personally don't care whether they like it or not unless they happen to be grading a paper of mine. In fact, I had a professor once who would deduct points from my college papers because I continued to use the neuter "he" even though she had asked me to stop using it. We even got into an argument over it once that spilled out into the hallway and ended up being mediated by another professor who was passing by and heard the quarrel; nevertheless, I still continued to use it and she continued to deduct points. I got a C in that class, but I learned from it. I had another professor after her who objected to the neuter "he" as well, but I had learned from my past mistakes so, just for her class, I patronized her by writing "he or she" and never argued with her about it. But, if you want it to be gender neutral, you can just use the neuter "he" and the "he" refers to both males and females much like "ils" (French masculine "they") refers to either a group of men or a group of men and women whereas "elles" refers to a group of only women:
Everybody sees what he wants to see.
Should this rule be too gender specific for you, you can always write it in the plural:
All of them see what they want to see.
They all see what they want to see.
Here's a link to another answer of mine that I wrote yesterday on the same subject: What pronoun should i use if i want to indicate something in general.