An indispensable training tool is the countdown timer. This allows everyone in the class (esp. us!) to be fully aware of time remaining before/ between sessions and on breaks. You run the countdown timer on your computer and the countdown is projected onto the screen for all to see. Here are a few I have used:

Online — must have live internet connection:
http://www.online-stopwatch.com

Mac only —
Minuteur 3 (free) or 5 (paid): http://phg-home.com/index_mac.html

Windows only —
BlingClock (30-day free trial, $20 after): http://www.blingclock.com
TimeLeft (free but you can’t make timer fullscreen): http://www.timeleft.info