The easiest way I've found is to use a non breaking space (ASCII code 255) in combination with the paragraph tags: <para>
To insert the non breaking space, press ALT + 255 (from the numeric keypad).
Here's an example:
/// <summary>
/// <para>This method does everything.</para>
/// <para> </para>
/// <para>...and I mean everything.</para>
/// </summary>
private void DoEverything() {
And this is how it looks in IntelliSense:
