Watch the Slide Vibrato: Shakes & Whips online guitar lesson by Lee Roy Parnell from Slide Guitar Slinger
Let's talk a little bit about vibrato when playing slide. Some vibrato is so wide that you can throw a dog through 'em, which can be effective, similar to Muddy Waters' vibrato technique. Personally, my vibrato isn't so wide, often times coming at the end of a phrase. I'm using it almost like a punctuation mark.
Something that I didn't realize I was doing was pulling sharp to create tension, then resolving it by going the other way. I also "whip it" at the end of a phrase — and there's no one way of doing this, just feeling it out for the moment in the song, keeping the audience on their toes.