Though it sounds counter-intuitive, one of the most effective communication skills you can possess involves not talking, but listening. Active listening is mentally engaging with what the other party is communicating, analyzing it, and adapting your own communication to naturally follow the flow of conversation. So often it's easy to get wrapped up in your own ideas, and how you're going to express them, that you can quickly tune out the person you're talking to, in turn missing key pieces of information. Take time to hear every word that the other person says, and quickly and silently develop something you can say in response to one of the other person's ideas.
Going hand-in-hand with active listening, appropriate body language lets you feel more comfortable while communicating, and it also lets the other person know you are engaged with what they say. Maintaining eye contact gives the impression that you are authoritative on what you are saying, and the other person knows you are listening to them. Limited and natural hand gestures present you as an animated speaker who is passionate about his topic, while maintaining good posture and keeping a relaxed face communicate to others that you are confident and friendly.
Maintaining an assertive tone and attitude while communicating is a necessary tactic for persuasive speaking. People who sound confident sound more knowledgeable, and therefore more people will believe you, taking the action you advise. Keep your tone natural throughout the conversation, putting emphasis on words important to your ideas. Don't let yourself speed up---a natural tendency when you're nervous---and enunciate everything clearly. Don't be overaggressive in attitude, but stay lively and friendly.
Obviously, a major part about communicating your ideas effectively is knowing and understanding them as well as you want the listener to know and understand them. Run a practice mental dialogue in your head before communicating your ideas out loud. Give yourself different scenarios on how the other person could respond, and try to anticipate potential radical turns in the conversation. Also, the more you prepare, the more you will be able to answer questions about your ideas, and more importantly, it will give you the confidence needed to keep your nerves under control and to speak assertively.
Effective communication skills are especially important in stressful situations when you need to deal with a negative issue. First, never let your emotions overshadow the issue at hand---don't yell or curse to express your anger, but get in a breathing pattern that will keep you calm. Try using "I" phrases instead of focusing on the other person, and don't bring personal and more importantly, irrelevant, judgments into the conversation. Try reiterating what the other person says---it shows you are actively engaged in the conversation, and steer everything towards a resolving the issue, not blaming or guilt tripping.