Maybe i am the last one to learn how to do it, but in case i am not, bellow are the details.
Google recognizes the following breadcrumb properties which can be labeled using microdata.

Property Description
title The title of a breadcrumb.
url The URL of a breadcrumb.
child The next breadcrumb in the hierarchy. The child must be another Breadcrumb item.

Let suppose you have the following HTML code describing a breadcrumb trail.
<a href="http://www.example.com/dresses">Dresses</a> --> 

<a href="http://www.example.com/dresses/real">Real Dresses</a> --> 

<a href="http://www.example.com/dresses/real/green">Real Green Dresses</a>

Then, the same HTML code marked up with microdata will be:

<div itemscope itemtype="http://data-vocabulary.org/Breadcrumb">

  <a href="http://www.example.com/dresses" itemprop="url">

    <span itemprop="title">Dresses</span>

  </a> --> 

</div>  

<div itemscope itemtype="http://data-vocabulary.org/Breadcrumb">

  <a href="http://www.example.com/dresses/real" itemprop="url">

    <span itemprop="title">Real Dresses</span>

  </a> --> 

</div>  

<div itemscope itemtype="http://data-vocabulary.org/Breadcrumb">

  <a href="http://www.example.com/clothes/dresses/real/green" itemprop="url">

    <span itemprop="title">Real Green Dresses</span>

  </a>

</div>

For more details, including how to do the same with RDFa, you may check Google's page.