Watch the The Slide Itself online guitar lesson by Lee Roy Parnell from Slide Guitar Slinger
It's important to talk about the slide itself, and the journey I've gone through finding what works for me. When I started out, slides weren't commercially available. You could break off the neck of a wine bottle and grind it down, as we'd heard the old blues guys did, or cut off a piece of pipe and use that.
For me, Corriciadan Cold Medicine bottles were the first thing I used, as did Duane Allman and others. However, through the years they began to change how they made them, and suddenly there was a "seam" on the bottle that if you hit it, you were in big trouble. And, on the road, it's hard to keep track of slides. People want them, and they'll flat out take them off your amp! So, I thought that I needed to find something that was commercially available in every music store in America. That's when I landed on the Dunlop 213, which is big, thick, and has a lot of resonance. I like it because it isn't tight on my finger, like the Corriciadan bottles were.
There are a few other slide types to be aware of: You can use a ceramic slide, with a little more texture, or a brass/metal slide, depending on what you need for your playing situation. By and large for me, the Dunlop 213 is the way to go!