Decide what message you want to deliver to your team. Do you want to get them to play together better. Maybe you want to emphasize team unity and sacrifice or you just want to squeeze more emotion out of them to play better. Decide what you want to say and get your team to do before you plan your speech. Make sure your speech speaks to those areas and don't include too many points in your speech. Just make sure you deliver your main message and be yourself. Your players will not respond if they think you are putting on an act for them.
Know what type of motivation your team responds to best. As a coach you should know what your team best responds to. Your team may often need a tongue lashing to get them going. Other team may be more sensitive to criticism so a more gentle approach is best. Deliver your speech in the way that will be most effective with your team.
Use stories and examples to help motivate your team. Players often respond well to a story or illustration rather than a long speech. They are often distracted with their own performance anxiety and have short attention span. A well timed story will help the athlete relate to what you are saying.
Time the delivery of the speech to maximize its effect. Often a coach will give a short speech right before the game to inspire the players. Decide when it is the best time to give your speech to be most effective. It may be after practice or even at halftime of a game.
Be brief but speak from the heart. The best sports motivational speeches invoke emotions in the players that cause them to perform better on the field. To invoke emotion in your players you must speak with emotion and from your heart. Believe in what you are saying to convince your team.