TheOptimizationIdiot - 10:42 pm on Mar 9, 2013 (gmt 0)
<h3>Remembering My Summer Vacation</h3>
and define the specific attributes of size, boldness, etc of <h3> in the header.
As an additional idea in HTML5 there's a element for that not requiring search engines to pick up on the styling differences to figure out it's a sub-heading.
I would keep in mind there's a bit of browser flexibility for how to present the sub-heading to visitors but in my experience browsers follow the display setting defined via css instead of choosing their own styling, but either way it might be worth looking in to or using in some situations.