So you've got your shiny new website but visitor numbers are not wonderful. You 'decide' as you sell widgets that you want to be on page one of Google for people who search for 'widgets'. However, you do some research and discover there are over two million pages in the Google index that contain the word 'widgets'. To compound this all the sites on the first two pages of Google are authority sites with good PageRank and lots of inbound links. They will be tough to beat.
Enter the long tail (kind of), but before we consider the long tail let's first consider Keyword Effectiveness Indices or KEI's. A keyword or key phrase's KEI is a measure of how useful the word or phrase will be to you. If a keyword or phrase is used by searchers AND appears on many web pages chances are the KEI will be a small number, not useful. However, if a phrase is used by searchers but doesn't appear on many or indeed any web pages the KEI will be a large number, useful.
In simple terms you want your site to contain words and phrases searchers use but other websites don't and this is where the long tail comes into its own.
Back to our widgets. You have found competition for the keyword widgets is intense. You are going head to head with some heavy hitters and it could be a long slog getting anywhere. It's time to use your head.
Using Wordtracker or similar research key phrases containing the word widgets. Doing this you will perhaps find people actually search using 'big blue shiny widgets'. Your research shows you that this phrase has a good search volume but very few pages competing for it. This means the phrase has a high KEI. Bingo. Create a page, optimise it for your phrase and watch your stats package.
Of course you don't just do this once, you create lots of new content pages optimised for the various high KEI phrases you have found.
Now this isn't actually long tail search, you can't optimise for the long tail, but it's a useful way of finding very useful and powerful long keyword phrases that might be easier to rank for than the phrases at the head.