Depends on whether we're talking about arranging accompaniment for songs, or arranging for solo guitar.

If the latter, a guitar (and guitarist) can usually handle three parts in an arrangement - bass, mid and high lines. Often the bass sets the basic chord, the mid is some kind of continuo and the high is the melody, but a good arrangement will often cross lines over each other, for example taking the tune into the middle line and having the high line become a harmony or counterpoint. I play Ashokan Farewell in DADGAD tuning, and the melody line for that goes over nearly two octaves so it might be in the high line at some points and down to the bassline at others, so it's a case of working out at each note which line the melody has gone into, and what the other lines should be doing around it to keep the tune moving. (And remember that rests are also a valid note if the tune doesn't need extra help at that point. :-)

For the technical side of arranging, remember that the same note can be played further up the neck on different strings, and often on open strings - open strings can give wonderful opportunities for harp-like runs up or down when you're playing above the 5th fret. Remember that sus2 and sus4 chords can give a nice effect, and open strings again are a good solution here. If you're reasonably familiar with altered tunings, remember that you can change your base tuning to make certain combinations of notes or fingerings possible (whilst also making other combinations impossible, so use with caution). Try playing with a capo and using different chords because that'll give a different tone, different chord inversions and different options for all those little hammeron and pulloff riffs, eg. if you're playing in G, try with capo 5 and playing D chords instead.

The same applies for accompaniment, but usually you're now looking at playing two parts on the guitar - bassline and continuo - whilst the voice supplies the melody line. Again the continuo can become a full-blown harmony or counterpoint, or it can just be a fingerpicked arpeggio to establish the background chord. And between verses or at other points where the singing stops, you might add some kind of lead line on the guitar (some kind of hook riff, possibly), so we're back to three active lines again.

Graham.