Basic Principle: A pronoun usually must refer to something earlier in the text (its antecedent) and must agree in number--singular/plural--with the thing to which it refers.

1. Remember that the indefinite pronouns anyone, anybody, everyone, everybody, someone, somebody, no one, and nobody are always singular.
   
2. Remember that this rule can create gender problems. If one were to write, for instance, "A student must see his counselor before the end of the semester," when there are female students about, nothing but grief will follow. One can pluralize, in this situation, to avoid the problem: "Students must see their counselor before the end of the semester." Or, one could say "A student must see his or her counselor. . ." Too many his's and her's become annoying, however, and the reader quickly becomes more aware of the writer trying to be conscious of good form than he or she is of the matter at hand.
   
3. Remember that trying to conform to the above rule can lead to a great deal of nonsense. It is widely regarded as being correct (or correct enough), near the end of the present century, to say "Somebody has left their bag on the floor," but many people would object its being written that way (because somebody is singular and their is plural). There is a great deal to be said, however, for using the word their as the gender-non-specific, singular pronoun.
   
4. Remember that when we compound a pronoun with something else, we don't want to change its form. Following this rule carefully often creates something that "doesn't sound good." You would write, "This money is for me," so when someone else becomes involved, don't write, "This money is for Fred and I." Try these: This money is for him and me. This arrangement is between Fred and him. Those are both good sentences.
   
5. To choose correctly between who and whom, re-phrase the sentence so you choose between he and him. If you want him, write whom; if you want he, write who. Who do you think is responsible? (Do you think he is responsible?) Whom shall we ask to the party? (Shall we ask him to the party?)