In daily conversation we can get very lazy. We speak quickly, cut off the ends of our words and end up pushing our words together. For example, when is the last time you said, "How are you doing?" Most likely you said, "Howyadoin'?" Sometimes it just takes too much energy to speak properly!
When it comes to word endings, the most common omissions are "ed," "ing" and "s." It is especially important that you remember to articulate "ed" and "s" endings because they are also grammatical markers. "Ed" marks the simple past tense and "s" identifies either plural nouns or third person singular verbs (see my recent post on subject/verb agreement). If you don't pronounce these endings it sounds like you are, at worst, making simple English grammar mistakes, and at best, like you're linguistically lazy and don't pay attention to detail.
Have you ever heard the rule "Write the way you speak?" Well, I think the opposite is also true. "Speak the way you write." You would never write, "I walk down the street," when you meant "walked." Articulate the entire word and you will be a lot easier to understand and will make a much better professional impression.