If you have the time for it, spend three months in England and it will stick to you without too much effort. I believe there are langauge programs of several months. while you are there learning the language you get a temporary job and they make sure there's enough interaction with native speakers. This is the best solution if you want a genuine English accent... You might want to contact the British embassy on this.

TRY to atleast watch bbc news hour channel daily 2 hours atleast.
then some good english movies!

You have to be born here

Very often, BPO firms serve American as well as British clients. So, having a neutral accent allows an employee the flexibility of switching from an American client to a British one and vice versa. And in communication, the emphasis is on achieving a neutral accent, removed from the vernacular tinge. An essential quality that the BPO employers look for is a neutral accent that can be easily `Americanised' or 'Britainised'. http://www.neutralaccent.com/neutralaccent.php

The are a few things you can do but the main point is to mimic people have the British accent. Here are some ways:
-Make British friends and have them help you with certain pronunciations
-Mimic speakers of the British accent from TV shows/Movies
-As the others have suggested, listen to the BBC radio and copy some lines that they say
-Go to London!

Good luck!