Are you producing a thesis, and trying to obey regulations that were drafted in the typewriter era? Or are you producing copy for a journal that insists on double spacing for the submitted articles?
LaTeX is a typesetting system, so the appropriate design conventions are for "real books". If your requirement is from thesis regulations, find whoever is responsible for the regulations, and try to get the wording changed to cater for typeset theses (e.g., to say "if using a typesetting system, aim to make your thesis look like a well-designed book"). (If your requirement is from a journal, you're probably even less likely to be able to get the rules changed, of course.)
If you fail to convince your officials, or want some inter-line space
for copy-editing, try changing \baselinestretch -
\renewcommand{ may be enough to
give officials the impression you've kept to their regulations. 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.
For preference, however, 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 has the advantage that it
switches off double-spacing at places where you would want it to
(footnotes, figure captions, and so on); it's very troublesome to
achieve this if you're manipulating \baselinestretch yourself.