The notion of "proper English" can be difficult to define. There are native speakers of English all over the world, and non-native speakers of English outnumber native speakers 3 to 1. Regional dialects of English are as varied as the speakers themselves. Yet, most English speakers know "improper" English when they hear it. Speaking proper English does not always mean following a strict set of rules. Instead, it is important to be understood and to know how to speak accurately, clearly and appropriately according to the situation.
Native Speakers of English
Pay attention to your surroundings and try to adjust your speech to the people around you. Avoid slang and cursing in formal situations like job interviews, public speeches and business meetings. Not all people understand the same slang terms, and not all people are comfortable with swear words. Using slang and cursing when there are more standard word choices available can make you look uneducated or impolite.
Familiarize yourself with the basic grammar rules you learned in school, and pay attention to how you use them. Everyone makes mistakes, but try to avoid the most common grammatical errors by paying attention to subject-verb agreement, conditional verb forms, correct verb participles and the difference between adjectives and adverbs. Avoid using double negatives and the word "ain't" if you're trying to impress people with your speaking.
Enunciate your words clearly. If your words are slurred or you don't move your lips and tongue enough when you speak, it can sound lazy and many people won't understand you well.
Read often, and read a variety of texts to improve your vocabulary and sense of sentence structure. The English most people use in daily life is fairly limited. You can greatly increase your knowledge of the language and become more comfortable with new words and phrases through extensive reading.
Avoid using words whose meaning and pronunciation you are unsure of. You might be misunderstood altogether, and using a word incorrectly can make people lack confidence in your intelligence and abilities. Look up the pronunciation in the dictionary, and pay attention to how a new word is used before using it yourself.
Non-Native Speakers of English
Improve your vocabulary, grammar and pronunciation by listening to English often. If there aren't many native speakers or advanced speakers where you live, listen to English-language music and radio, and watch TV and films. You can also improve your language skills by reading. Read as much as you can, and read both formal and informal English in newspapers, books, magazines and on the Internet.
Pay attention to individual vowel and consonant sounds and try to notice how they're different from your native language. Also listen to the intonation and rhythm of English and do your best to imitate it. A non-native speaker with good intonation and rhythm will be better understood than someone with perfect grammar who speaks unnaturally.
Practice speaking as much as possible, even if it's just with your friends who are also learning English. Speaking helps you learn to get your mouth around tricky words and phrases. It also helps you acquire language permanently so that it becomes automatic, and you can start speaking more fluently instead of thinking about every word.
Get a good grasp of English grammar, but don't focus too much on it. Many students of English spend all their time worrying about grammar details and don't develop their other skills like vocabulary, listening and speaking.
Ask people to correct you. Often non-native speakers make mistakes they aren't aware of, so asking other people for help can make you more aware of how you sound.
Remember that your main goal in communicating with others is making yourself easily understood. Most people expect mistakes from non-native speakers, and they don't care about them unless they can't understand you. If you're worrying about being perfect and feeling embarrassed for every mistake, you're probably speaking too slowly with too many pauses. It will be harder to understand you, and you will seem to lack confidence.