To recoup your cost per click advertising (Google AdWords, for example), the most important is to start by choosing the right keywords on which to bid, so as to create keywords which generate qualified traffic to your site. For a list of relevant keywords for your business, here is a list of 5 tools to use.
The first three tools are accessible from the Google AdWords in the "Opportunities" and then clicking "More Tools" tab.
Insights for Search
These tools allow you to compare search volume trends for periods, categories and regions. You can compare keywords to see which ones are the most sought after by users by period, and by region. The tool is particularly interesting to study the seasonal cycles that have an important impact in many sectors. Insights for search are also relevant to study search volumes by region. Finally, it provides you with search terms similar to those you want to audit: which help you to complete your start list.
Keywords Generator is accessed off AdWords interface at: https://adwords.google.fr/select/KeywordToolExternal and the new version is available when you log into your Google AdWords interface. The principle is simple: you enter a list of keywords and / or phrases and the tool will return a list of results including keywords that are considered the closest by Google. The results will inform you about the number of monthly searches on Google for each keyword, and the AdWords advertiser competition for that keyword.
This is the most useful for retrieving synonyms and phrases similar to those you have selected in your list of starting tool.
Traffic Estimator Tool
Only accessible when you are logged into your AdWords account, this tool will provide you with estimates of the number of clicks, cost per click (CPC), to ad positions and daily cost. This information will be valuable to set your daily budget and will allow you to see the priority of each keyword.
I will talk very quickly about Google Trends because interest seems limited because I consider it a "light" Google Insight. It provides researched trends with volumes expressed as indices, an overview of the distribution by region, city and language. So I do not really see what insights can be obtained for search, except for the ability to use trends for websites to get audience websites estimates, but for keywords, this has no value.
Use keywords wide
Your list of keywords should never be definitive, you must constantly improve and monitor changes in the behavior of internet users. The tools presented above will help you to do this but the most effective method is to use keywords in broad match mode in your campaigns.
By defining your campaigns you want to use one (or more) keywords in broad match, Google will show your ads on queries with synonyms, variations in the masculine, feminine, singular and plural variations including errors typing, spelling, grammar, words from the same lexical field, etc ...
Thus, you can at progressively extract new keywords typed by users through the report of search queries and use mode exact phrase or exact keyword in your campaigns. This method is more efficient because unlike the tools presented above, it is not based on historical trends but the queries that users really capture the moment. Which provides information for keywords with low traffic.