Google Adwords team created a series of Adwords optimization tips for advertisers. One of my favorite posted article from their blog was about brainstorming on new keyword ideas. Here are the following tips on how to brainstorm on new keyword ideas.

• Google Trends helps an advertiser to look at the search volume for different keyword ideas. You can use Google Trends to figure out how users are searching for it. Google Trends also shows how frequently your topics have appeared in Google News stories, and in which geographic regions people have searched for them most.
• Search Query Report shows performance data for the search queries that triggered your ads that received clicks. Since Search Query Report includes search queries and their corresponding performance data, you can use this report to fine-tune existing keyword lists. Specifically, you can use this report to identify both new keywords and negative keywords that you’d like to add to your account to better control when you would like your ad to appear.
• Forums and Wikipedia. Browsing different discussion forums is a great way to keep track of all the buzz related to your industry. Discussion forums related to your industry and online information sources like Wikipedia are other ways to learn how the general public perceives your product or service. If there are misconceptions about your industry, you can use this information to your advantage.
• Google Search. Finally, use Google search to your advantage. Perform Google searches to see what the most relevant results are for both natural and sponsored search results. You can check out the sponsored results to see what other people are advertising and use natural search results to find sites you can browse for new ideas.

Looking at these results can also help you brainstorm new negative keyword ideas so that you can better target your audience.