For myself longtail keyword research/analysis/targeting is more of an art than anything else. You basically review your set of primary keywords/phrases and come up with as many variations (using synonyms, adding adjectives and the like) to come up with a longer list.

Then eventually whittling it down by determining how much traffic each of the keywords/phrases would produce and whether its worthwhile to pursue (prioritizing).

With longtail keyword analysis you have to look at the whole set really and determine for the 100 longtail phrases how much traffic will I receive? Keeping in mind that for each set of phrases you may need to create specific optimized pages for them...resulting in quite a few extra pages for the site.

Good luck!