There is an American saying, “Slow and steady wins the race.” Simply put, you will have more success if you take your time and do something slowly, rather than hurrying and making mistakes.
How does the above saying relate to speech? The slower you speak, the easier your speech will be to understand. A rapid rate of speech will make your accent sound heavier and your speech more difficult to understand. Slower speech will help you think about the sounds and words you are saying and will allow more time for your tongue and lips to reach the proper placement for those sounds. The objective is not to speak too slowly; it is to speak as slowly as you need to in order to be able to pronounce sounds more accurately and clearly. When speaking slower, remember to keep your words connected, unless it is a natural place to pause. Speaking more slowly means prolonging vowels, not separating words.
Don’t worry about what Americans think about you when you are speaking slowly. The goal is to make your speech understood. Try to speak slowly as much as you can in all speaking situations, especially when speaking on the telephone. Speaking on the telephone is the most difficult speaking situation because your listener must rely only on what they hear. They don't have any visual cues to aid in understanding you, such as watching your lips and body language, or seeing your facial expressions.
There is much more to oral communication than just speaking slowly, but until you can receive accent reduction classes, this is a good place to begin, and is a very quick way to help improve your speech right away!