I have tried to learn slide off and on for a little while now and always became really frustrated because I just could not play it clean at all. I ways always using a pick and thinking I could get by like that.
But then I started to watch Derek Trucks play a lot more, and I saw the way he uses his right hand - incredible! His right-hand muting technique is a thing of beauty.
I have started working on open E tuning and I have really been focusing on my right hand technique. I am taking things slow and concentrating on getting my thumb to mute the bass strings and my pinkie and ring finger to mute the high strings, and just picking with my index and middle fingers. Focusing on my right hand for the last couple of weeks has REALLY helped me get a little more satisfaction out of slide and has ended the frustration.
The other thing that helped me a lot was to go up in string guage from 10s to 11s. The high E string with 10s was really hard to get any feel with the slide. Having an 11 for the high E helps that a lot.
This guy has some really nice videos that have helped me quite a bit. I have not bought any of his DVD's, but his free Youtube videos are really good. Here is a link to one general video on Slide in Open E:
YouTube - Learning slide guitar: A slide guitar lesson about Open E