Make your readers feel as if they were right there with you. Where were you? Who was with you? When was it? What time of day was it? What happened? What were you feeling? What could you see? What could you hear? What could you smell? Using the five senses often helps to make a description more vivid. Also, use specific words. For example: Instead of saying "dog," say "golden retriever."
One way to check out whether or not your description was effective or not is to ask people to draw a picture of it. If they draw similar pictures to what you witnessed, then you know you did a great job.