I've read countless books to learn how to improve my own communication skills. The one thing that has been consistent thoughout all the books i've read to date is the importance of repetition. Essentially - if you want to communicate something you have to say it right and say it again. That's right, say it right and say it again.
Finding new ways to repeat the same message is both tricky and essential. For instance, if you read Beyond Bullet Points from Cliff Atkinson you will see that the key to 'stickness' is to repeat the same message throughout multiple mecanisms (words, pictures...etc). You can check out this very interesting webinar in which Cliff talks about his methodology. Other great resources about Cliff's methodology here.
A recent and interesting book might be Words That Work by Frank Luntz. The concept of repetition, tying a message to an image or inviting people to envision your message are explored.
The book also talks about the basics of communication: filters. You might say something but people filter what they hear. Being an efficient communicator is about knowing what the right thing to say is, repeating it but also knowing how to say it right in relationship to the audience...
Ultimately - if you want to test your communication skills, you can always visit www.slideshare.net. This site hosts powerpoints and often runs ongoing contests!
