NICs, DHCP and IP addresses

I am the sysadmin in a small company, where we build a lot of PC-/workstation type of computers. They usually come with several LAN ports as well as WIFI on the motherboard, and because the machines are moved around and disconnected a lot, they are rarely connected in the same way every time. This leads to a few, often occurring nuisances, namely that they change IP addresses - I try to set them up in DHCP with a single address based on the MAC address, and then add them to DNS, and this breaks it, of course. I have tried to set up DDNS, but couldn't get it to work, and getting my estimed colleagues to use just the same LAN port every time appears to be just outside their capabilities; they may simply be too highly trained to focus on such mundane matters. I have also tried to set up up several MAC addresses with the same IP and hostname, but the DHCP daemon (this is on Linux) very sensibly refuses to allow it; I suppose I could change the MAC addresses on the interfaces so they are the same on the machine, but that seems an unsound thing to do. So, what can I do to tame the situation a bit? What is the best way to make sure that people don't come to me, complaining that they can't connect to humbaba, which they've just started up?