A quick and easy way of getting inter-line space for copy-editing is
to change \baselinestretch - \linestretch{1.2} (or,
equivalently \renewcommand{) may
be adequate. Note that \baselinestretch}{1.2}\baselinestretch changes don't take
effect until you select a new font, so make the change in the preamble
before any font is selected. Don't try changing \baselineskip:
its value is reset at any size-changing command so that results will
be inconsistent.
For preference (and certainly for a production document, such as a
dissertation or an article submission), use a line-spacing package.
The only one currently supported is setspace (do not
be tempted by doublespace - its performance under current
LaTeX is at best problematical). Setspace switches off
double-spacing at places where even the most die-hard official would
doubt its utility (footnotes, figure captions, and so on); it's very
difficult to do this consistently if you're manipulating
\baselinestretch yourself.
Of course, the real solution (other than for private copy editing) is not to use double-spacing at all. Universities, in particular, have no excuse for specifying double-spacing in submitted dissertations: LaTeX is a typesetting system, not a typewriter-substitute, and can (properly used) make single-spaced text even more easily readable than double-spaced typewritten text. If you have any influence on your university's system (for example, through your dissertation supervisor), it may be worth attempting to get the rules changed (at least to permit a "well-designed book" format).
Double-spaced submissions are also commonly required when submitting papers to conferences or journals. Fortunately (judging by the questions from users in this author's department), this demand is becoming less common.
Documentation of setspace appears as TeX comments in the package file itself.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=linespace