Watch the Tones and Gear online guitar lesson by David Hamburger from Focus On: Standard Tuning Slide
There are a few essential factors to a good slide tone, and they all intersect with one another when you play. You’ll need to tweak your instrument slightly in terms of string gauge and action - not a lot, just enough to get a little more resistance from the setup. You’ll need a slide that’s thick enough to provide some density - nothing super thin and plinky. And then, well, there’s your hands. With the right setup and a decent slide, the rest depends upon developing your left and right hand muting.