Try using fingerpicking when playing slide guitar. Most slide players play this way because you get better control and you can mute the strings with the picking hand.
If you are having trouble getting a good sound, you may want to get thicker strings or raise the action on your guitar. Raising the action means literally raising the strings higher off of the neck so there is a lower chance of the strings hitting the frets. Be careful! If you raise the action up too high, you won’t be able to fret the strings when you go back to playing normally. Even worse, you might offset the tension with the truss rod, so make sure you are experienced or working with someone who is trained as a Luther.
Use vibrato extensively when playing slide. Vibrato gives your slide playing a personal touch and feel. You can create specific moods based on the type of vibrato you are applying.
Listen to slide guitar greats like Muddy Waters and BB King in order to hear how their
style and trademark vibratos create their definitive sounds, then try to mimic them.