Here is an alias that I've used often to view packet payloads using tcpdump which filters out all the overhead packets (just contains payloads).
I usually stick the following lines into my .bashrc on all the servers I install.
You can pass as argument the interface you want to listen on (defaults to eth0) via a '-i eth0:1' for example. It snarfs in the payload, so it's easy to follow what's going on.
An equally viable alternative is to install tcpflow.