Double line spacing

Simple: put

\usepackage{setspace}
\doublespacing
% or:
%\onehalfspacing

into your preamble. (TeX-FAQ advises for setspace and against doublespace.)

The setspace package does it for you, but it turns doublespacing off within footnotes and floats like figure and table captions. That's usually desired.

But if you don't want to use setspace, perhaps because of the mentioned reason, you could use the command \linespread, for instance:

\linespread{1.5}

A package may be preferred over such a command though.

The simplest possible way is probably by using the plain TeX macro \openup

E.g. if you want double line spacing, add a single line-height to the line spacing (1em) using:

\openup 1em

(don't use any braces around the argument, the macro takes its argument as if you had written \openup=1em, i.e. an assignment of a dimension).j

Following that macro's invocation all lines will have a single line's height extra to separate them, later on you can revert this effect by giving the negative argument:

\openup -1em

This macro works by increasing (\advance) the three parameters (\lineskip, \baselineskip and \lineskiplimit) that govern line spacing by the given amount. It's defined in plain.tex if you want to have a look at it.