I'm sure that most of you know Google Translator. Well, I think you can used to improve your pronunciation. Now, you are asking how to do it.
First of all you have to use Google Chrome.
Secondly, surf to the url : http://translate.google.es/?hl=es&tab=TT
There is a microphone at the bottom left. Click on this icon and speak loud and clear.
For example, say " I've never been in London". If you do well, you'll see it at the screen.
After that, you can compare your pronunciation doing click on the speaker icon.