When I was studying to take my SANS GCIA exams, I found I needed a nice reference diagram showing the layout of the headers of TCP and UDP packets. On the internet you'll find many printable diagrams for this but they all seem to be the same format as the diagram in Steven's famouse TCP/IP book. I found this was hard to use because the diagram doesn't match the format of the output generated by tcpdump and I was mostly using the diagram to compare tcpdump output. So I made my own header diagrams that match tcpdump's output and made it so it was one page with a large font. A good quick reference.
You can get these from my download area. The files are packets.sxw (OpenOffice format) and packets.pdf (Adobe Acrobat format). Derek Armstrong has provided a version of the packet header diagrams that includes ICMP and is in color! (RTF Format).