Live and love. Real life situations make the best songs. When Bobby Womack sang about love, it was clear he didn't hear about it somewhere. Life is the best inspiration for a song.
Write down experiences in a daily journal. Refer to them often and keep them in chronological order. Looking back on these will provide song ideas.
Pick a title for the song first. Refer to the daily journals for a title. Decide on a mood that goes along with the title. Remember that mood and maintain it throughout the song.
Create music and lyrics at the same time. Writing the words without the music makes the process disjointed. Everything should flow.
Steal from the greats. There's nothing new under the sun. Don't feel bad about getting an idea from a stack of old soul records. Just make sure it's an original creation when it's all said and done.
Practice this process over and over again. Eventually each songwriter develops his or her own style for creating a song. Don't be afraid to experiment with new ideas, and remember that R&B is rooted in decades of joy and pain.