There is also a nice feature of Google Analytics that can reveal phrases that bought you traffic and what page they rank on. You need to set up a new profile and filters but it can be really insightful.
As Peter says when you work across the site properly the cascading effect can elevate longer tail keywords from deeper pages naturally.

Good post… As I said in the rest of my talk, we normally would specifically focus the optimisation of the websites we work on a core set of highly searched root phrases, but in doing so, find that the variations of longer tail traffic driven, ultimately:

a) drive as much, if not more traffic than the highly searched, competitive phrase

and b) due to the more specific nature of the query and the resulting answer of said query from the website that this traffic converts at a much higher level.

Most of the time I dont find we need to specifically target the long tail – if you are doing the SEO right in the first place by concentrating efforts on improving the websites rankings for those root phrases, it has a cascading effect on the number of keywords that Google and the other search engines make that site visible for.