Let's expect you already have Munin installed and working and you want to reduce disk I/O and improve responsiveness by adding rrdcached... Here are the complete steps to integrate rrdcached:

Basic Installation

First install the stock package 
apt-get install rrdcached
and integrate it with Munin:
  1. Enable the rrdcached socket line in /etc/munin/munin.conf
  2. Disable munin-html and munin-graph calls in /usr/bin/munin-cron
  3. Create /usr/bin/munin-graph with 
    #!/bin/bash

nice /usr/share/munin/munin-html [email protected] || exit 1

nice /usr/share/munin/munin-graph --cron [email protected] || exit 1
    and make it executable
  4. Add a cron job (e.g. to /etc/cron.d/munin) to start munin-graph: 
    10 * * * *      munin if [ -x /usr/bin/munin-graph ]; then /usr/bin/munin-graph; fi

The Critical Stuff

To get Munin to use rrdcached on Ubuntu 12.04 ensure to follow these vital steps:
  1. Add "-s <webserver group>" to $OPT in /etc/init.d/rrdcached (in front of the first -l switch)
  2. Change "-b /var/lib/rrdcached/db/" to "-b /var/lib/munin" (or wherever you keep your RRDs)
 So a patched default Debian/Ubuntu with Apache /etc/init.d/rrdcached would have 
OPTS="-s www-data -l unix:/var/run/rrdcached.sock"
OPTS="$OPTS -j /var/lib/rrdcached/journal/ -F"
OPTS="$OPTS -b /var/lib/munin/ -B"
If you do not set the socket user with "-s" you will see "Permission denied" in /var/log/munin/munin-cgi-graph.log 
[RRD ERROR] Unable to graph /var/lib/munin/
cgi-tmp/munin-cgi-graph/[...].png : Unable to connect to rrdcached: 
Permission denied
If you do not change the rrdcached working directory you will see "rrdc_flush" errors in your /var/log/munin/munin-cgi-graph.log 
[RRD ERROR] Unable to graph /var/lib/munin/
cgi-tmp/munin-cgi-graph/[...].png : 
rrdc_flush (/var/lib/munin/[...].rrd) failed with status -1.
Some details on this can be found in the Munin wiki.