Create a page description less than 160 characters, preferably split into two equal length sentences, although not vital. If the counter goes past zero then you've too many characters and Google will likely truncate your description, which won't look as neat in search results.
Choose 4-8 keywords and/or keyphrases that best suit your web page. To add a keyphrase i.e. character counter simply add a hyphen between the words so it becomes character-counter. This will then be correctly formatted on output. Note: Google places zero emphasis on keywords at this time, but it can't hurt to include them anyway.