My most common slide is up to a unison note with the next string up, as a way to emphasize that note. If you're looking for a spot to try this, look for an emphasized open D, A, or E, then instead of just hitting the string, start a note or two below and slide into it. You can slide into other notes, but it's not as fun.
Tunes in D and A often start with those notes, so you can use a slide to kick off a part, but even G tunes offer good slide material.
It's easier to explain with an example. In the 'B' part of "Yankee Doodle," a G tune, there are a couple of spots where slides will work.
Yankee Doodle keep it up,The italics in the second and fourth lines are strong D notes. If you count out the eight beats in each line, the last word of the previous lines are spoken on the seventh beat. The eighth beat is free to start a slide.
Yankee Doodle dandy.
Mind the music and the step.
With the girls be handy.
I'd start these slides from the B note (second finger on the G string), right on the beat and slide up to the D, slowly enough to arrive at the seventh fret just in time for the first note of the next line. I don't play that next note on the seventh fret, though, I play it on the D string, just the way I usually would. In other words, my left hand's arriving at the closed D at the same time my pick's arriving at the open one on the string above it.
There are lots of variations on this little ornament. A slide to start a tune's a good kick-off, especially if you're changing keys. A slide into the very last note of a tune gives you a dramatic ending -- having that pair of unison notes gives the last note more sustain, too -- lets it ring longer.
My favorite variation, though, doesn't change where or when, it changes what: what instrument, that is. Watch banjo players, who use slides to good effect a lot, and you'll see how it's done. Fiddlers do it, too. Me, though, I love slides when I'm playing backup guitar, as an alternative to bass runs. Guitars have an open A, an open D, and even an open G that you can slide into to kick off a part or to end a tune. Once in a while, I'll play two, or even four, in a row instead of a chord.
I don't see other backup guitar players do this, so perhaps they don't like the sound as much as I do. For me, besides sounding good, it just feels great.