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.