pt-deadlock-logger

There are a number of options to run the deadlock logging tool but there is an actual example:

pt-deadlock-logger h=my_logging_server.com --user=USERNAME --password=PASSWORD --dest h=my_server.com,D=percona_schema,t=deadlocks,u=USERNAME,p=PASSWORD --create-dest-table --daemonize

This is the most useful free way I've found to capture deadlocks in MySQL.