pronouns - Is "none" singular or plural? How can I decide?

It can be both, depending on the subject that goes with it.

From http://www.grammarmudge.cityslide.com/articles/article/1026513/9903.htm:

A common misconception is that none
must always be treated as singular.
The customary support for this view is
that none necessarily means "not one"
(implying singularity); in fact,
"none" is just as likely to imply "not
any" (implying plurality). As noted
in The American Heritage Dictionary:
"the word has been used as both a
singular and a plural noun from Old
English onward. The plural usage
appears in the King James Bible as
well as the works of John Dryden and
Edmund Burke and is widespread in the
works of respectable writers today."

The most sensible rule is the one that
governs similar words designating a
portion of something (fractions,
percentages, and indefinite pronnouns
such as some, most, many, all, and
more). Just as we write "some of it
is" or "two-thirds of it is", we would
write "none of it is"; just as write
"some of them are" or "two-thirds of
them are", we would write "none of
them are."

Idiomatically, few of us would be
comfortable with "None these people is
happy" or "None of my friends is going
with me." The sense here is plural:
not any. Yet the myth of the
singularity of none persists, even
among people who frequently say, "None
. . . are." (Why is it that some
people cling to a simplistic rule,
even when it's wrong, rather than face
the necessity of making a choice based
on sense?)

When the sense is plural (as indicated
by a plural noun or pronoun in the
following prepositional phrase – "none
of [plural entity]"), none is plural;
when the sense is singular (as
indicated by a singular noun or
pronoun in the following prepositional
phrase – "none of [singular entity]"),
none is singular.

Futhermore, we may have some instances
in which either is correct. The
American Heritage Dictionary notes:
"The choice between a singular or
plural verb depends on the desired
effect. Both options are acceptable
in this sentence: None of the
conspirators has (or have) been
brought to trial." And that is true
because the sense of none may be
construed here as either "not one" or
"not any."

Two last points are worth noting. It
is difficult to avoid treating none as
a plural when it is modified by almost
– "Almost none of the children were
[not was] well-behaved." And, in
constructions such as "None but a few
of the students were able to complete
the test," none must be treated as a
plural.