How do you keep track of which IP addresses are used on your network?

You should setup a DHCP server, and then keep a spreadsheet or table in a doc of any static addresses you manually set. We have a page in our documentation wiki, but a Google spreadsheet should be good too.

Is there something you have against a DHCP server? I tend to believe that DHCP pretty much always makes my job easier and I try to use it whenever possible. If you want computers to reliably get the same address setup reservations.

In my experience what works best is the Master IP Spreadsheet to Rule All Others. That is, there exists one spreadsheet and it is everyone's responsibility to update it. If it ain't in the spreadsheet, it ain't live. No ifs no buts. People soon learn to update it. Also, I wouldn't use DHCP for any servers, even test ones. The last thing you want on a server is the chance that it's IP might change.