If you're using FireFox, I don't think there is an easy way. The problem is explained with brilliant simplicity here by one Blake Stephens:
To quote:
The title that's automatically generated from hovering over a marker
is actually the standard HTML title attribute, which works the same
way as the alt tag attribute on an image. It's not exactly easy to get
multiple lines on one, since browsers decide for themselves when to
get the line to wrap (so really long alt texts and titles can be
readable, they wrap). I don't believe it's possible to forcibly insert
a line break.
However, this does not stop you from circumventing the generic title
text altogether and creating your own that's completely stylable. You
can attach an even to the marker for the 'mouseover' event and
generate a div tag filled with your content that follows the mouse
around; removing/destroying/hiding itself when the 'mouseout' event
fires. That seems like the most reliable way to me.