How To Improve Your Pronunciation:

1. English is considered a stressed language while many other languages are considered syllabic.
2. In other languages, such as French or Italian, each syllable receives equal importance (there is stress, but each syllable has its own length).
3. English pronunciation focuses on specific stressed words while quickly gliding over the other, non-stressed, words.
4. Stressed words are considered content words: Nouns e.g. kitchen, Peter - (most) principal verbs e.g. visit, construct - Adjectives e.g. beautiful, interesting - Adverbs e.g. often, carefully
5. Non-stressed words are considered function words: Determiners e.g. the, a - Auxiliary verbs e.g. am, were - Prepositions e.g. before, of - Conjunctions e.g. but, and - Pronouns e.g. they, she
6. Read the following sentence aloud: The beautiful Mountain appeared transfixed in the distance.
7. Read the following sentence aloud: He can come on Sundays as long as he doesn't have to do any homework in the evening.
8. Notice that the first sentence actually takes about the same time to speak well!
9. Even though the second sentence is approximately 30% longer than the first, the sentences take the same time to speak. This is because there are 5 stressed words in each sentence.
10. Write down a few sentences, or take a few example sentences from a book or exercise.
11. First underline the stressed words, then read aloud focusing on stressing the underlined words and gliding over the non-stressed words.
12. Be surprised at how quickly your pronunciation improves! By focusing on stressed words, non-stressed words and syllables take on their more muted nature.
13. When listening to native speakers, focus on how those speakers stress certain words and begin to copy this.

Tips:
Remember that non-stressed words and syllables are often 'swallowed' in English.
Always focus on pronouncing stressed words well, non-stressed words can be glided over.
Don't focus on pronouncing each word. Focus on the stressed words in each sentence.

Source: http://esl.about.com