Watch the Standard Tuning Slide: 5 online guitar lesson by Robbie Calvo from SoloCraft
As you can imagine there are many different types of slides on the market from glass to brass to stainless steel. A couple of things to remember are that the weight of the slide is very important. The slide rests on the strings and glides on them. No pressure is needed so the weight of the slide determines the amount of contact with the strings to a large degree. I prefer brass slides for electric playing as they are heavy and produce a really nice warm tone. As you can see I am using a brass slide that allows my finger to protrude out of the top. This will allow me to fret notes without removing the slide. I also use a glass slide but mainly for acoustic playing and Dobro. Glass sounds smoother on acoustic instruments to me. So, my point is - try out a bunch of different styles and materials before settling on anything.
The finger on which you place the slide is very important too. I use the slide on my little finger as this allows me to use the other fingers to fret notes and chords. There are many incredible slide players that use the slide on the first, second or third finger. This isn't wrong, it's just a preference but will impede chordal playing at times. Bonnie Raitt uses the slide on her 2nd finger (correction; I apologize, I said 3rd finger in the video) and she is amazing as you know!
The slide should be kept parallel with the fret wire, and the notes for correct intonation are now found directly over the fret wire not in between the frets. For example, to play a D note at the 3rd fret on the 2nd string you would place the slide directly over the 3rd fret wire. Get it? Try that now and reference the pitch with the fretted note. Keeping the remaining fingers of your left hand lightly resting on the strings directly behind the slide will prevent unwanted overtones from ringing out especially at high volume and gain levels. If unsure about what I am saying here please watch the video closely and follow what I am doing.
Just a quick note about string gauges and guitar action: Heavier gauge strings are preferable as this will give you better contact with the slide. Having a slightly higher action will allow you to glide over the fret board with the slide without "clipping the frets". Try using your standard guitar and setup and see if you can achieve satisfactory results without making any changes or using another guitar. Ultimately you may want to have a guitar that you use specifically for slide. I use the Nunley Custom Tele exclusively for slide and it has been set-up with 10 gauge strings. I normally play 9s for regular guitar playing.