I am surprised that in this old question nobody has shown the most common (and quite simple in this case) sed command:
$ sed -i '1s/^/new_text\
Which works in most shells and is portable to several sed versions.
If GNU sed is available, you may use this:
$ sed -i '1s/^/new_text\n' file_name
The difference is that GNU sed allow the use of a
\n for a newline and others need a literal newline preceded by a backslash (which also work in GNU sed anyway).
If a shell which accepts the
$'…' syntax is in use, you may insert the newline directly, so sed sees that the newline is already there:
$ sed -i $'1s/^/new_text\\\n/' file_name
Which works for more sed versions.