Press lightly on the strings with the slide. Downward pressure isn't necessary to get a good tone and actually impedes it. To get an exact tone, you need to land on the fret wire.
Drag other fingers behind the slide as you drag the slide downward. Laying your fingers on the strings blocks a lot of unwanted sound that the strings make when you play the electric slide guitar.
Adjust the fingers up and down behind the slide to find the tone that you like. If you want to add some funk in your music, you might like the overtones that the fingers block. Try running a few chords with and without the fingers dragging behind.
Pick the notes exactly like you normally would but use the slide to add some pizazz to your guitar playing. This gives a lot of diversity when you play electric slide guitar. You can create some interesting sounds by adjusting the type of slide that you use.
Get some vibrato by using the slide. All you need to do is raise and lower the slide above and below the fret wire. You can create a lot of different effects by speeding the rate of movement up and down.
Add some great licks by using regular chord patterns and choosing some slide patterns in between the chords during a riff.