Watch the Sliding 6ths online guitar lesson by TrueFire from Play Blues Guitar 3: Tools & Techniques
In the Allman Brothers version of "Stormy Monday" one of the main "hooks" you hear is the sliding 6ths. This is a sound that is used in tons of different blues progressions so it is really important to be able to master it. Make sure you experiment on moving it around to different positions on the neck. You can also play around with using this over different feels and tempos and with various chord shapes. As with much of the blues - learn it, and then make it your own!