Today I learned about (the incredibly well-supported) <wbr> element. Use it where you want the browser to know it can introduce a line break.

Take the longest word in a major English dictionary:

<p>pneumonoultramicroscopicsilicovolcanoconiosis</p>

And break it.

<p>pneumonoultramicroscopic<wbr>silicovolcanoconiosis</p>