Choose a few songs that really highlight your talents. If you are known for having amazing vocal range, pick something that showcases your abilities. If you play an instrument, make sure you choose a song that works with that particular instrument. Try to find a local recording studio so your demo has a professional feel to it. Put your best foot forward when you approach managers. Consider hiring additional musicians to work on your demo to give the music the full R&B feel.
Whether you work with another band or form one of your own, get comfortable in the live performance setting. Even playing small bars and clubs will help you build your confidence and create a stage presence. Once you have sent out your demo music, you can invite managers to come and watch you perform. This helps give the manager a better idea of what you are capable of performing.
When you are recording your demo and working with other musicians, ask for recommendations regarding managers. You can look in local phone books or in cities with big musical centers, such as New York or Nashville. No matter what you find out about managers, ask around to see what other musicians' experiences have been as far as working with certain individuals. Target those managers who come with good recommendations. Be sure a manager accepts unsolicited material prior to sending a demo. If the manager does not, consider inviting the person to one of your live performances and have a demo ready to give him should he attend. You can also send an inquiry letter introducing yourself and why you would be a good client to have.
After you have invited a manager, sent an inquiry and/or sent a demo, make sure you follow-up. It is easy for things to fall through the cracks, so it remains important for you to keep in contact with the office. While it might feel like you are being bothersome, being persistent tends to increase your chances of being listened to. If you are given a flat-out rejection, you can still send updates occasionally as major events happen in your career, such as sales numbers online or new shows where you're performing.