I prefer glass also. In addition to placing the slide directly over the frets, muting of the strings not being sounded is critical to effective slide technique. I wear the slide on my middle finger, and use my index finger to mute the strings. You can wear the slide on any finger that feels comfortable for you, but you must be able to mute. I also mute the strings while fingerpicking with my picking hand. I don't use a pick for slide. This technique was used by Duane Allman (THE slide player, imho), and is how he achieved such a fluid, clean sound. The picking fingers rest on the strings to mute them; the required string is finger picked, and the finger is then replaced on the string at the end of the note's duration. This is a difficult technique to master but when done correctly really makes a slide solo sound great. The real trick to slide playing is to use open tunings (D G and E are most popular) . The open tunings bring the solo patterns much closer together; you don't have to move the slide as much as you would with standard tuning.