Normally I'd just bookmark this, but the link has already died once and I had to dig it up from alternate development trees.  Current source: https://github.com/autostatic/scripts/blob/rpi/jackstart.  And this is the rather fascinating page that lead me to all this.

(note: I'm not sure this is compatible with the latest Linux kernel, and it's not been tested. YMMV.)

#!/bin/bash

## Stop the ntp service
sudo service ntp stop

## Stop the triggerhappy service
sudo service triggerhappy stop

## Stop the dbus service. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
sudo service dbus stop

## Stop the console-kit-daemon service. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
sudo killall console-kit-daemon

## Stop the polkitd service. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
sudo killall polkitd

## Only needed when Jack2 is compiled with D-Bus support (Jack2 in the AutoStatic RPi audio repo is compiled without D-Bus support)
#export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/dbus/system_bus_socket

## Remount /dev/shm to prevent memory allocation errors
sudo mount -o remount,size=128M /dev/shm

## Kill the usespace gnome virtual filesystem daemon. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
killall gvfsd

## Kill the userspace D-Bus daemon. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
killall dbus-daemon

## Kill the userspace dbus-launch daemon. Warning: this can cause unpredictable behaviour when running a desktop environment on the RPi
killall dbus-launch

## Uncomment if you'd like to disable the network adapter completely
#echo -n “1-1.1:1.0” | sudo tee /sys/bus/usb/drivers/smsc95xx/unbind
## In case the above line doesn't work try the following
#echo -n “1-1.1” | sudo tee /sys/bus/usb/drivers/usb/unbind

## Set the CPU scaling governor to performance
echo -n performance | sudo tee /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

## And finally start JACK
jackd -P70 -p16 -t2000 -d alsa -dhw:UA25 -p 128 -n 3 -r 44100 -s &

exit
If you don't mind killing the networking, you can add these to the script as well:

sudo service ifplugd stop
sudo killall ifplugd
sudo service networking stop