I love the sound of the 9 (it's the second of the chord, meaning the note next to the root, but one octave higher)
In major chords
Basically, for an A major, you remove the finger on the third string, and for the D, you remove the finger on the high E string...
For other chords, it's trickier (the E involves quite a stretch, but is the best sounding)
------0---
-----0----
----1-----
---4------
--2-------
-0--------
that's the Eadd9, and it's a cool one...
I learned those notes listening to "for the love of god" by steve vai (it opens with a beautiful arpegiated E minor add9 on a guitar/sitar)