In addition to Ham and Bacon's answer (universal "he" and singular "their") you can also do the following:
(polite) imperative to avoid mentioning the actor: "Please, click the button to proceed."
passive tense to avoid mentioning the actor: "Button must be clicked to proceed."
use "one", though this is more appropriate for formal texts then for user instructions: "One must click the button to proceed."
Also, from the free dictionary, on usage of she there is an interesting observation that it can be and is used as a generic she:
Usage Note: Using she as a generic or gender-neutral singular pronoun is more common than might be expected, given the continuing debate regarding the parallel use of he. In a 1989 article from the Los Angeles Times, for instance, writer Dan Sullivan notes, "What's wrong with reinventing the wheel? Every artist has to do so in her search for the medium that will best express her angle of vision." Alice Walker writes in 1991, "A person's work is her only signature." It may be argued that this usage needlessly calls attention to the issue of gender, but the same argument can be leveled against generic he. This use of she still carries an air of unconventionality, which may be why only three percent of the Usage Panel recommends it in sentences like A taxpayer who fails to disclose the source of ? income can be prosecuted under the new law. · Some writers switch between she and he in alternating sentences, paragraphs, or chapters. This practice has been gaining acceptance, especially in books related to fields like education and child development, where the need for a generic pronoun is pervasive. It can also be seen in academic journals, where the sentence The researcher should note that at this point in the experiment she may need to recheck all data for errors might be followed later in the same section by The researcher should record his notes carefully at this stage. This style may seem cumbersome, but if generic pronouns are required, alternating between she and he can offer a balanced solution in an appropriate context.
There are many style guides lying around on the net and wiki has relatively nice overview.