When I was at school, we were told to start letters with Cher Monsieur if we didn’t know the gender of the person we were writing to. At the time, I believe the rule was to assume masculine until proven otherwise.

This behaviour mimics the fact that a mixed plural is always masculine too (Paul et Marie sont grands whereas Paul est grand and Marie est grande). The saying goes le masculin l’emporte sur le féminin (masculine wins over feminine).

Similarly, it is usual to refer to the reader as le lecteur ; for example le lecteur intéressé se réfèrera au livre de Jules Dubonnet (The interested reader should consult the book by Jules Dubonnet).

However, there are people who believe^{[citation needed]} this is a way of asserting a male superiority and that it’s unfair towards women. The saying le masculin l’emporte sur le féminin in itself it particularly eloquent and can be understood as “men win” if taken out of a pure grammatical context (which it shouldn’t be).

As such, if you want to keep a neutral stance, it may be better to either mention both (Chère madame, cher monsieur) or maybe use a plural (Les lecteurs intéressés se réfèreront au livre de Jules Dubonnet) which, even if it’s still masculine, tacitly encompasses female readers too.

In French we are not as surprised if “une personne” relates to a man and if “quelqu’un” is a woman. (Yet feminine words like “personne” which may easily refer to a man are quite rare, currently I can just think of “sentinelle”, “estafette”, “recrue”, “ordonnance”, all four from the military vocabulary BTW, “personnalité”, “victime” and “connaissance”; there are probably a few other expressions like “grande gueule”).

Even if you try to be less obvious with masculine terms (using on, the plural, the second person, …), the rules of French are such that the masculine will still be present. Trying to be real neutral is harder and more non natural than in English.

It is perhaps why this matter is less sensitive than in English, at least from a male perspective (a female view would be welcome). The convention is to use the masculine; doing otherwise is more often than not a political statement.

Note that while the rule le masculin l’emporte sur le féminin is often stated and used, there is a quite strong usage to use feminine name for professions mostly feminine (for example, people will use institutrice to refer to the teacher of unknown sex for small children, the use of infirmières to refer to the whole profession is also common).

When you’re writing a guide, you can choose from four options when talking to the reader:

• imperative form:
  Cassez les œufs et battez-les.

• infinitive form:
  Casser les œufs et les battre.

• impersonal form:
  Il faut casser les œufs et les battre.

• passive form:
  Les œufs doivent être cassés et battus.

The rules for general context are very well described in the other answers, but I just wanted to add something for mailing rules (from where I work, but I assume we aren’t the only mass printing company with such rules):

• known gender (M) ⟶ Cher client,

• known gender (F) ⟶ Chère cliente,

• unknown gender ⟶ Chère cliente, Cher client,

Here, the need to be affable with your customers (who are more often female than male in many activities) has changed the slightly aged rule of Le masculin l’emporte sur le féminin…