This is according to Monique Harrisberg CEO and founder of The Voice Clinic, a communication training company. "First of all, don't worry about not having a native-English accent. It's important to be able to speak clearly, so that people can understand you. There are many things that you can do to improve your pronunciation and speaking skills," says Harrisberg.
Some useful tips to help improve your pronunciation is to listen to spoken English as often as possible. Listen to how speakers pronounce various words and phrases and 'model' your pronunciation on what you hear. It would also help to use the phonetic alphabet page (at the beginning of most good dictionaries) as a guide to pronouncing new words.
Harrisberg says word stress is also important. "In fact, it is more likely that someone misunderstands you because of wrong word stress than because of the wrong pronunciation of a sound. Be aware of intonation and sentence stress. Not all words in a sentence have equal stress, and generally only the nouns and verbs are stressed. Sentence stress isn't 'fixed' like word stress. In fact, you can stress words that are normally unstressed in order to highlight different meanings," she says.
Lastly, if you speak too fast, the danger is that you could skip over some words, fail to pronounce them completely, or mix them up. If you speak too slowly, you might end up sounding unnatural. But it's better to speak slowly and clearly than too quickly.
