1. Observe and imitate the mouth movements of those who speak English well
You can watch from Youtube, movies, or real people from the real world near you who speak English well. Don’t be ashamed to look silly, or to be mocked by the people around you. You’re the one who will get better in speaking English correctly in the end.
2. Learn also the correct intonation and rhythm
Different English accents have different intonations and rhythms. I suggest you learn the standard accent first, i.e. standard American accent. Why do I suggest standard American? Because Indonesian people are easier to imitate the Americans because of the Hollywood movies, so it’s easy to find sources to learn the standard American.
3. Don’t speak too fast when you’re still learning
If you speak fast and you make mistakes in the pronunciation of some words, you’ll look stupid and people won’t understand what you say if you speak fast. But don’t speak too slowly, either. You don’t want people to think you’re a robot or mental retarded, do you?
4. Read aloud in English for at least 10 minutes daily
Find an English novels, short stories, news, anything. Read them aloud along with the intonation and rhythm. Reading aloud for ten minutes daily is enough to help you improve your pronunciation and lose you native accent.
5. Install a speaking dictionary on your computer
This is very practical rather than buy a dictionary. Why do I say this is practical? Because the technology advancement (notebooks, netbooks, even tablets) that makes your life a lot easier. You can even install a speaking dictionary on your smartphone (of course you must use the mobile version of the dictionary). I recommend you install Concise Oxford English Dictionary. When you install it on your device, you can look up for a word, click on the speaker logo, and voila, it pronounces the word!
6. Record your own voice and find the pronunciation mistakes
You can do the number 4 tip and you record it while you’re reading. Listen to your recording, and find the pronunciations that you think you need to improve.