A slide is a metal/glass/ceramic tube which fits over a finger (most commonly the ring finger or little finger, but any will work). If you wish to experiment with slide guitar, but do not have a slide, objects ranging from lighters and glass bottles to sections of metal pipe and batteries can work just as well, and in some cases provide entertainment and stage presence to a performance. Do not press the string down. The slide rests on the string, not enough to give fret buzz, but enough to stop the string buzzing against the slide. Some players will lightly deaden the string behind the slide with a trailing finger to stop any unwanted vibrations.
Practice getting a crisp note without sliding first. Because the slide rests on the strings, the slide playing a single note should be directly above the fret, not behind it as with the fingers. Usually the slide guitarist keeps the slide moving backwards and forwards slightly to create a vibrato effect.
A common technique found in slide guitar is playing fingerstyle as opposed to the use of a pick or plectrum. The benefits of fingerstyle playing includes the ability to more easily pick the desired strings, while using the other fingers to dampen the other strings from undesired vibration. Raising the action of the guitar is also recommended. The normal low action, which is ideal for playing lead in standard tuning, is counter-productive when playing slide because of string buzz and lack of a clear sounding note. For this reason many guitarists have a second guitar where they raise the action to such a height to make it almost unplayable using normal technique. This high action guitar is permanently kept in a "open tuning" and is used exclusively for slide playing. Note that raising (or lowering) the action means that the intonation of the guitar has to be re-set. This can simply be done with a guitar tuner and just involves turning the string adjuster until the open string and its octave at the twelth fret (frettted and harmonic) produce exactly the same note. Basically the needle/display of an electronic guitar tuner should settle exactly dead center regardless of whether you are playing an open high e string or fretting its octave at the twelth fret. A guitar that is correctly set up will show this on all strings. Setting the action of electric guitars is very easy due to the string adjusters however acoustic guitars have their intonation set at the factory and don't have string adjusters. Adjusting the action of an acoustic should be left to a guitar shop or luthier who specializes in repair and maintenance.
Though slide guitar is often played in open chord tunings, Open G and Open D being the most common, playing slide in standard tuning is also possible and can add a new dimension to your playing. Slide guitar has always provided a fascinating approach to playing the guitar and the sound of the slide has found a home in genres such as Rock and Country.
One of the earliest mentions of slide guitar is in W.C.Handy's autobiography "Father Of The Blues":
"As he played, he pressed a knife on the strings of the guitar in a manner popularized by the Hawaiian guitarists who used steel bars"
This is also one of the earliest references to the blues. As you can tell from the quote above the use of the slide in no late-comer to the blues genre and there is large body of work from the 1920s to the present day. No guitarist can confuse the slide playing of Duane Allman with that of Robert Johnson. Each period informs of itself the dictates of taste and style.
Robert Johnson is cited as the first great slide guitarist. Other famous blues performers had comer before him, Blind "Lemon" Jefferson was a major entertainer during the 1920s but Robert Johnson is considered to be the first major exponent of the slide. During his life-time he only recorded a handful of tracks and though known locally for being a fine entertainer; the world-wide fame that is associated with his name now is more down to later blues fans and guitarists who have sought the roots of the blues.