Treat line breaks as line breaks.  Best practice: line continuations made with backslash at end of line.  This rule is optional If implementing a mixed native/creole mode.  Also see [List of Line break markup].

Creole:
{{{
This is the first line.
This is the second line.
}}}

Recommended XHTML:
{{{
<p>
This is the first line.<br/>
This is the second line.
</p>
}}}

Output:

This is the first line.\\
This is the second line.