Legend has it that a blues guitarist while boarding a train heard a local man playing a guitar in a strange voice-like, wailing manner. He noticed that the man was using the blunt end of a butter knife to fret the notes. The man boarded the train and took the technique with him and spread it around. Later the Coricidin medicine bottle became the standard for playing slide guitar, while some enterprising (yet impoverished) blues guitarists, broke the neck off of beer and wine bottles. Hence the term, bottleneck slide.
Since that time, slides have been made out of everything from chromed steel to glass to porcelain, and the technique has been moved from acoustic blues to southern rock and beyond.
Before you can start playing slide guitar, you'll need a slide. Aside from all the new types you can get, (ceramic, half size, movable etc etc) there are two main types, glass and metal. As a basic rule of thumb, glass is mellow and smooth, metal is harsher or brighter. Glass is also generally not as heavy as metal, so it has less sustain, a quality which can be both good and bad. Also, its important to keep in mind that most electric guitars, need very little help in the way of achieving a harsh tone, so the increased sustain of electric combined with the inherent bite, make glass slides more at home on electrics, and brass or steel slides more at home on an acoustic.
In reality, however, you don't even need to decide between the two. Slides are extremely inexpensive, so go to your local guitar shop and try a few on, find a size that fits you and buy one of each (read the next paragraph before doing so). You can experiment to find your favorite sound when you get home.
The next step is to pick a finger. Put the slide on either your ring, or little finger. Which finger you use is a matter of preference, some like to use the little finger, because it leaves three fingers open for fretting notes behind the slide, others prefer (like me) to use the ring finger, because it offers a little more control. Try both, pick one, and make it a habit.
The third item on the list of things to know before you begin, is tuning. Although its true that you can play in whatever tuning you like, some work better than others. Most people use an open tuning, like Open E for example. Open E is basically tuning your guitar so that when you don't fret any notes, it plays an E major chord. You can achieve this buy tuning up the 4th and 5th strings 2 frets, and the third string 1 fret. Drop-D tuning, allows you to play the 5th and 6th strings as a power chord, and use the remaining strings in scale patterns that you already know for licks.
I, personally, usually play in standard tuning and avoid playing more than 2 strings at a time because its easiest for me to play remember scales in a tuning I know. If your really inventive you could even tune to a 9th chord, minor chord, or any other form you could think of.
So there you have it kids, a second article explaining the more advanced parts of bottleneck slide will be up shortly, so practice these ideas while your waiting.