So you can't start using <section> and <header> yet. But that doesn't mean you can't start thinking like that.
What the HTML specs are really about is standards, and standards are just agreed upon practices. So if more people start using classes this way in preperation for html5 than we'll be halfway there.
Use HTML5 Semantics for HTML4
Look at the spec and if make sure you're using your tags like HTML5 suggests. Particularly b, i, em, strong, address, blockquote, dl, small.
<div class="section"><div class="header"><div class="footer">Arguably use
<date/><time/><datetime/>-> Use Microformats datetime-design-pattern
<abbr class="datetime data-1998-03-12T08:30:00-05:00">12 March 1998 from 8:30am EST</abbr>
<span class="progress"><span class="value">0</span>/<span class="max">10</span></span>
<meter/>-> Similar to
<progress/>. can also add
<kbd class="samp">sample text</kbd>
<div class="figure"><em class="legend">
If you want to feel a little better about this choice you can validate against HTML5.
While this doesn't really have much to do with HTML5, it is another way you can standardize your practices. There's lots of services that look for microformats so there's no reason not to learn and use them. They'll likely only get more important as time goes on.