homebrewでインストールしたらlaunchctl(macで自動起動を司るなにか)の設定ファイルがついてくるので、それを使うだけ。バージョン部分は適宜変更すること。
$ cp /usr/local/Cellar/redis/2.4.8/homebrew.mxcl.redis.plist ~/Library/LaunchAgents/
$ launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.redis.plist
$ launchctl start homebrew.mxcl.redis
homebrew.mxcl.redis.plistの中身
<?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>homebrew.mxcl.redis</string>
<key>ProgramArguments</key>
<array>
<string>/usr/local/bin/redis-server</string>
<string>/usr/local/etc/redis.conf</string>
</array>
<key>RunAtLoad</key>
<true/>
<key>UserName</key>
<string>marqs</string>
<key>WorkingDirectory</key>
<string>/usr/local/var</string>
<key>StandardErrorPath</key>
<string>/usr/local/var/log/redis.log</string>
<key>StandardOutPath</key>
<string>/usr/local/var/log/redis.log</string>
</dict>
</plist>
**参考リンク
http://www.maruko2.com/mw/LaunchDaemons_(launchctl,_launchd.plist)_%E3%81%AE%E4%BD%BF%E3%81%84%E6%96%B9
https://github.com/mxcl/homebrew/pull/10913