launchd launch agent for squid installed via Homebrew, place in ~/Library/LaunchAgents.

net.wezm.squid.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>KeepAlive</key>
  <true/>
  <key>Label</key>
  <string>net.wezm.squid</string>
  <key>ProgramArguments</key>
  <array>
    <string>/usr/local/sbin/squid</string>
    <string>-N</string>
    <string>-d</string>
    <string>1</string>
  </array>
  <key>RunAtLoad</key>
  <true/>
  <key>UserName</key>
  <string>wmoore</string>
  <key>WorkingDirectory</key>
  <string>/usr/local</string>
  <key>StandardErrorPath</key>
  <string>/usr/local/var/log/squid.log</string>
</dict>
</plist>