If you haven't used TestFlight, you should start immediately. It lets you distribute ad hoc builds easily to testers. You only need to archive in Xcode and a TestFlight helper app will prompt you to enter release notes as well as who should be given access to the build. Forget about managing your own FTP and HTTP servers or upgrading plist files for your own hosted ad hoc builds. Testers click in an email and download your test build.
Did I mention it's free to use?