Travel is often one of the best ways to improve your skills in a foreign language. Can you spend time in England or the United States? "Total immersion" in the language (being surrounded by native and fluent speakers at all times) can be one of the most effective ways to build memory (and muscle memory) of a new language.
Travel is very expensive, though, and requires time off from work or school. Another good option is consuming lots of media in the target language. Movies, TV, and radio will give you a range of accents and styles to choose from. Watch and listen to them frequently.
Try watching a short BBC news report (for example), and then filming yourself giving the same report. Listen to the recording of yourself and compare it to the BBC newscaster's report. Try to pick out the differences between your accent and the newscaster's. Then, do your recording again. Is it closer to the native speaker's?
This takes time and effort. It's a matter of habit and muscle memory, so repetition will be very important in improving these skills.