Long Tail Pro is a very useful piece of software for long tail keyword research. One of its best features is that you can carry out heavy analysis on the keywords of your choice, without being limited to only working with one or two at a time. It can provide you also with vast numbers of longtail keywords, or you can apply filters to find the precise keywords to fit your bill. And you can check your rankings at the same time!
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 limit on manual keywords could be higher. I personally wish we could input 10,000 keywords at a time, instead of 200. However, I understand the costs that Long Tail Pro has to maintain each time a new manual keyword is input. Not a deal breaker, I just wish the limit was higher. (To be fair, I don't think most keyword tools have a bulk manual option at all).