Watch the Slide Mechanics online guitar lesson by Andy Aledort from 30 Beginner Slide Blues Guitar Licks You MUST Know
Before we get into the licks, I'd like to focus on the specifics of how to develop proper slide technique in regard to the necessary techniques for the fret and the pick hand. The first thing is to set the "action" of your guitar - the height of the strings relative to the fretboard - higher than you may normally use. Many slide guitar players will set their action higher when playing with a slide because it's easier to sound a note by placing the slide against the string without danger of pushing too low and banging into the frets, and with more space under the string, it's easy to move the slide up and down the strings to produce clean and even-sounding notes and a clear, sustaining vibrato. The correct amount of pressure to use with the slide is just enough to sound a note while being careful not to press the string down against the fretboard.
Another aspect to address is the type of slide to use: glass, metal, ceramic, or another material; each of these materials will produce a different tone. I prefer glass "bottles", the type used by Duane Allman. Another concern is the finger on which to wear the slide - most players use either the pinky, the ring finger, or the middle finger. I prefer the ring finger, as did Duane Allman as well as Derek Trucks, Warren Haynes, and Ry Cooder.
The slide needs to be positioned directed above the fret wire for the note to sound in tune properly. It will take practice to master proper slide technique so all of the notes you play are perfectly in tune. Lastly, a slide vibrato is produced by moving the slide up and down the length of the string in evenly repeated increments. The optimal slide vibrato will emulate the sound of the human voice.