How to improve your accent skills if you don't have a natural ear for accents
There are a number of resources you can go to in order to improve your accent skills, and I have no doubt that all will help you get to where you need to for whatever accent you need.
Firstly if you are still at drama school, talk to your voice teacher to see if they have any information that may help, be it a ‘short cut’ into a certain accent if you need to prepare for one for a part (just isolating certain sounds may help, i.e. singing ‘I’ve Got a Brand New Combine Harvester’ if you need a West Country accent – try it, it works!), or if they have any worksheets or books that might be useful to look at.
Further to this, there are plenty of accent workshops available at the various centres for actors – indeed The Actors Centre, The Actors Guild, The Actors Temple are the ones I know of and are some of the best, but there may be more – take a look online; all of the above have websites. There are also specialist voice centres who do courses, workshops, and recordings such as voicereels especially for voice actors. The misleadingly titled The Showreel is one I know of and they do accent courses, but again there are others out there. Again, have a look online.
If you are really having trouble, if might be worth getting in contact with a voice and speech tutor. They will be able to offer you one on one, very specific work, and should really be able to help if you really are desperate. Check out the book Contacts, published by Spotlight, or by The Actors Yearbook who should have some listed, but if not again search online, or get in contact with one of the top drama schools and they may be able to put you in contact with one of their voice teachers, or can point you in the right direction.
One of the best resources though that I know of, that I have used countless times in the past, will use in the future and am actually using for work right now, is a website call The International Dialects of English Archive. This amazing resource - produced by The University of Kansas - is free, and provides voices and their subsequent accents and dialects recorded from all over the world, from the U.S., Austria, Iraq, New Zealand; truly, you name it, they got it. They have both sexes, various ages, and accents from many parts of the various countries too, for example they have around seven Boston recordings, as well as recordings from every other state in America. Typically in each recording the person who’s voice they use reads out one of two passages, which I suspect cover every sound that an English speaker makes, and then talk a little bit about themselves for a minute or so. As said the website is completely free, and although it is now a streaming only site so you can’t downloads the clips to play on an ipod or other, you can pick up the website on your phone and stream the clips easily there. Check it out, it’s truly a godsend for actors.