Not only videotaping yourself, but videotaping others' runs, then turning the sound off, and practicing at what point you should give the command. Obedience people, in particular are used to having an infinite amount of time to get their commands out and have a great deal of difficulty learning to think ahead.
Late commands are a bugaboo of many in agility, regardless of experience.
One good method is to have someone call out the commands while you run a mock course. That way you have instant feed-back on how you're doing. Another good method is to use a radio headset which offers feedback in the same way. Radio Shack has an inexpensive set that works well. You can hear yourself while you watch your dog run.