Watch the Breakdown online guitar lesson by Richard Gilewitz from Fingerscapes
Great effectiveness with the slide can often times, particularly with the blues, occur when the left hand index finger lifts off the fingerboard from it's usual 'dampening' position by lifting it off the strings while the slide remains down. This can produce a great rattle or 'snarl' evoking the proper mood from the tune.
Preceded by right hand palm mutes can prove to enhance this effect greatly. Simply rest your right palm against the base of the strings in front of the bridge. As soon as you lift your right palm up slightly, try lifting your left hand finger off to allow for the shimmering sound - enhanced even more with some vibrato.