Many flamenco pieces specify striking the guitar to complement the melody or dancer's rythm. The most common technique is rattling fingertips onto the pick-guard area, but other methods include thumping the top plate below the bridge with the heel of your palm to get a low pitched thud, or flailing fingernails against the corner of the top and side to get a high pitched rattle.

Differently pitched sounds can be made all over the top of the guitar, and the manner of striking can include all of the basic conga methods. Just experiment until you find something that fits the music you're working with at the time. There are also a lot of percussive string techniques, such as striking quickly (and rebounding cleanly) with fingers or thumb. Usually near the bridge works better, but sometimes the click of strings against the frets adds to the desired effect. Even hammering on strings could be considered percussive to some extent.

This piece features several different percussions:

http://artists.mp3s.com/artist_song/499/499469.html

~S~