Effective communication skill does not necessarily mean your proficiency in any particular language, rather, it is your power that communicates your thoughts clearly.
Your sound knowledge of vocabulary of English language can hardly make you an excellent communicator.
Rather, a seeming thought process and your confidence level can help you to communicate even better. Like every other aspect, communication skill also needs a thorough practice. Even the most introvert person very frequently communicate with his friends, family members, teachers and many others. It may also happen that when he attempts to communicate with a new person, he becomes nervous and hardly remains at a state to express himself in front of the others.
As effective communication is a skill, it has to be earned through various modes.
You can start talking to yourself positioning yourself in front of a mirror. You can start streamlining your thoughts in a dream situation, which you may have to encounter in the remote future. Essentially, you need to start talking to others to convey your messages successfully. You may feel a bit shaky at the initial stage, but you can hardly deny that practice makes you perfect.
Your initial teething problems will be wiped off by the unabated storm of consistent practices.
You should also notice the people who sounds nice to you. May be, even the news reader of any particular channel may magnetize you. Try to understand, how he speaks and start enchanting them within yourself. You may follow some political leaders also.
When you are communicating across the interview board, you may have to bring some real life examples to make your communication more effective.
Whatever it may be, you need to practice and get prepared before the final count down.