Listen more carefully. As most communication professors, professionals and coaches will tell you, the key component of effective communication is listening. Most people are guilty of poor listening skills, feigning attention or actively distracting themselves with cell phones or environmental factors. However, if you give all of your attention to the individual sending the message, you are more likely to absorb more of the message presented, allowing you to comprehend and respond to it more effectively.
Ask questions. One of the biggest pitfalls in communication is making assumptions about the message being sent. If part of the message is ambiguous or unclear, take the time to rephrase what has been said in order to verify that you are understanding the message being presented. If you have a specific question, don't be afraid to ask it. As the saying goes, there are no stupid questions. If it allows you to better understand what is being said, you are more likely to be able to effectively communicate with the message's sender.
Be thoughtful in your response. It is all too easy to fire back a quick response to someone, whether it be via e-mail, text, or in person. However, in doing so, you may misspeak, confuse the listener, or miss a key component of the message that you are trying to convey. Take a moment to reflect on who you are talking to, the subject matter at hand, the environment in which the exchange is taking place, and the actual information you are trying to pass on.
Double check yourself. This is of particular importance with all written communication. Make sure to re-read whatever you are about to send in order to be sure about the content, phrasing, and basic grammar of the message. This not only helps you to avoid simple errors that would paint you as unprofessional, but will aid in your ability to specifically craft the message you're trying to send. In person, this translates into speaking a little more slowly than you usually might. This can help you pay attention to the non-verbal signals being sent by the message's recipient and adjust your phrasing or content midstream to accommodate their reaction.
Follow up. If you want to make sure that your message is understood, ask if the recipient has any questions. This may come in the form of another e-mail or text after enough time has passed for the recipient to read and understand your message. In person, it may be as simple as asking the person you're talking to if they understand or have any questions. This makes you a courteous communicator, and makes it more likely that the individuals listening to or reading your messages understand what you are trying to convey, making you more effective in the process.