The key to keeping your note ringing is to push your finger along the string at an angle similar to sweeping a floor, angled pushing rather than dragging. If your finger is at 90 degrees upright it will get stuck behind fret bars and you can lose your aim. Sliding downwards tends to lose a lot of volume from your string due to the physics involved so only use them briefly or with gain sustain.
String slides can be used for various different sounds / styles, for example forcing a fast slide the instant the string is played can give a strict and snappy attitude to your music whereas perfoming the slide towards the end of the note can give a more casual groove in joining notes.
Here's an idea: any songs you learn that use slides, try playing it with a slide style you heard in another song and see how it changes the..?
