mod_jk has a bevy of options that should, in general, be set in workers.properties in order to tune the connection to Apache as well as Tomcat. By default it's pretty wide open as far as connections and timeouts are concerned; it's very easy for Apache to accept way, way way more load than Tomcat sitting behind it can handle. Here are some of the important ones I've found and what I typically set them to, the actual reading of the docs is left up to you. :)

# Specify the size of the open connection cache.
worker.ajp13.cachesize=1
worker.ajp13.retries=3
# below are in seconds
worker.ajp13.cache_timeout=60
worker.ajp13.socket_timeout=30
worker.ajp13.recycle_timeout=60

# Advanced ping-pong options
# 0 (full recovery)
# 1 (don't recover if tomcat failed after getting the request)
# 2 (don't recover if tomcat failed after sending the headers to client)
# 3 (don't recover if tomcat failed getting the request or after sending
#    the headers to client).
worker.ajp13.recovery_options=0
# below are in milliseconds
worker.ajp13.prepost_timeout=5000
worker.ajp13.connect_timeout=5000
# under load it can take awhile to get a reply when doing an initial JSP compile
worker.ajp13.reply_timeout=60000