First few words from the topic in the Google Search Results

Presently it shows the global forum description below the title in the Google Search Results (for any word) for each viewtopic page spidered by Google. This is a big minus and it discourages the searcher to visit the forum site. How can I ensure that the Google Search Results will show a small extract of actually the text in the first post of the topic so that people are encouraged to visit my punbb forum?