Watch the Acoustic Slide Guide online guitar lesson by Richard Gilewitz from Acoustic Slide Guide
Many types of slides can be selected based on their physical composition (i.e. - glass, brass, etc). This is generally a player preference. Once you have selected a slide to your liking, consider the proper pressure needed when applying the slide as well as placement, position of all of the fingers on the slide playing hand, and the use of a dampening technique behind the slide for a smoother tone.