Usually, long-tail keywords are just a longer version of your existing keywords or key phrases. For example, let's say your keyword is "towing services". A long-tail version of that keyword would be "good towing services for cheap".
Use any keyword tool like Google keyword tool, Word tracker, Keyword discovery, etc. For example take Google keyword tool. Type in the phrase you want to get rank. Then you will shown some keywords list with their volumes. Now you need to pick up your keywords list from the volumes there. Try changing the keyword to get more set of list and choose your keywords(long or short) according to the volumes
In my point of veiw, your short keyword site url must has specified keyword such as nokiamobile(dot) com. this website has nokia keyword and also not very long.
put in the master keyword to the tool of your choice to see some variations and suggestions, then search the more refined keyword as your starting point.
install google webmaster tools on your site.. you will get stats of some potential long tail keywords...
If you are looking for a long tailed keyphrase make sure that it has your keyword or some variation of the keyword in it. That is how you can make long tailed keywords work for you easily.
Go to www.semrush.com , enter the interesting domain into the search line, then go to keywords report , export it into Excel and sort by position. You need positions from 10 to 20.
use google keyword tool you will get long tail keywords and check the global and monthly searches against the specified word.