Be sure to re-evaluate these keywords every few months -- once a quarter is a good benchmark, but some businesses like to do it even more often than that. As you gain even more authority in the SERPs, you'll find that you can add more and more keywords to your lists to tackle as you work on maintaining your current presence, and then growing in new areas on top of that.

Given you have a good idea of where to start and are fairly confident you are speaking the same language as your client, jump start research by generating related keyphrases and long tail variants with the ever so easy to use Google Autocomplete. This tool makes predictions based on what you are typing that are a reflection of Google search activity.
The primary difference is that Long Tail Pro is a big time saver over MS. With Long Tail Pro you are able to enter in multiple seed keywords at once (rather than just 1 with MS). So, instead of entering in 1 seed keyword, waiting five minutes, then going back and entering in one more; you can enter 5 seed keywords (for example), hit generate and then see all the keyword ideas at once!
This tool allows you to type in your keyword phrase or keyword in the field that is provided and it will generate a long list of suggested keywords.  It has three button level and you can perform a deeper search by clicking on button level 2. You will get a list of keywords or keyword phrases that are related to the original list provided in level one. If you want to continue further, you can click on level three and then click the link at the bottom to download the data. The tool can assist you to discover those keywords that your competitors could be ignoring and use those opportunities to create fresh targeted content.
Using the good ol' free Google Keyword Planner is great. But there is an alternate universe of Long Tail keyword phrases that live in Google's other database--namely, their "Autocomplete database." That's the database that is accessed when you're typing queries into the Google search box (notice that list of suggested keywords that pop up as you type?)
3) Google: This is pretty straight forward but it’s the main reason I like it. I search for my main seed keyword in Google, and use the keywords that Google itself highlights in bold on the search results, plus the “Searches related to” section at the bottom to get keyword variations or LSI. That’s basically what Google is telling you that topic is about. No need for a thousands other tools. I use these to optimize the on page of my target pages as well.
WordStream for PPC is a full-service PPC management platform for creating, optimizing and managing pay-per-click advertising campaigns in Google AdWords. Our PPC software includes easy-to-use PPC keyword tools, campaign creation and expansion tools, Quality Score optimization tools, and more to help marketers get their PPC work done quickly and cost-effectively. WordStream for PPC customers experience increased traffic and greater ROI.
Understandably – not everybody who buys Long Tail Pro will have the audience or platform to be able to promote as an affiliate. For example – a food blogger’s audience won’t care about a keyword research tool. So even if affiliate promotion won’t work for you – I hope I’ve shown how the tool will more than pay for itself if used for what it is intended for.

It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.