The key thing to remember here is that the slide must be placed directly above the fret to avoid sounding flat, and not behind where you would normally place your finger. The next issue is damping. Damp (mute) behind the slide by laying your first finger parallel to the slide on the strings, this controls unwanted harmonics. Damp in front of the slide by using a combination of palm muting and “finger guarding” (as Sonny Landreth describes it). This is achieved by resting your unused picking hand fingers directly onto the strings above the string you are playing to prevent them from sounding as you move the slide.