For some reason the existing solution for configuring server density to run on startup with OS X was failing for me.
For whatever reason, this is what ended up working for me, and I dare not touch it now!
Label com.boxedice.sd-agent.start RunAtLoad StandardOutPath /tmp/sd-agent.out StandardErrorPath /tmp/sd-agent.err ProgramArguments /usr/bin/python /usr/local/bin/sd-agent/agent.py start