What comic conventions have been used in Much Ado About Nothing?Give specifc examples.

Of course, the specific use of conventions would need to
reference the production in which they were used to answer your question completely. 
However, I will offer some suggestions of comic conventions that I have seen used and
reference the specific scenes in the play in which they were
used.

The two conventions that spring to mind right away
for me are pratfalls and
slapstick.  Both of these terms refer to types of physical
comedy -- a pratfall being a fall on one's backside (think slipping on a banana peel)
and slapstick being a term that includes most forms of comic violence between two or
more characters (think pie in the face).

The first scenes
that use these sorts of physical comedy are the eavesdropping scenes in which both
Beatrice and Benedick overhear that the other is "in love" with him/her (Act II, scene
iii and Act III, scene i).   Pratfalls suit the antics of Beatrice and Benedick as they
move about the stage from hiding place to hiding place, attempting to overhear the
conversations, yet trying to remain hidden.

The other
scenes where these sorts of physical comedy would be used are the two scenes with the
Watch.  Dogberry and Verges would be the main "clowns" in the play, but combining them
with the Watch provides much opportunity for comic violence or slapstick, especially
when apprehending Conrad and Borachio.  The scenes in which the watch appear are Act
III, scene iii and possibly Act III, scene v.

There are
multiple opportunities for physical comedy in Much Ado, especially
pratfalls and slapstick.