Skype released a great document clearly explaining how to optimize Skype perfomance safely on a LAN. It is available here. Skype's Guide for Network Admin.'s
There is other software that I have been working with that allows for much greater control of Skype on a LAN by managing bandwith and packets; allowing for optimization of Skype traffic. The market for this type of Packet and Internet prioritization software will continue to increase, as Network Admin.'s try to remove latency from VoIP sessions on the LAN.