Nice answer Dave. You really just have to try to see what, if anything, sets off the behavior. For instance, if you're trying to capture a yawn, pay attention to when your dog yawns. (Usually after playing, waking up, sometimes training, exercise, etc.) If you've been sitting around for hours waiting for her to yawn and she's still ready to play, go for a jog. When you get home, give her a good massage. She should be ready to snooze right away and let out a big yawn! Lol. Mudflap is very quiet, not at all vocal. This has proved to be very tricky when teaching her speak. Many people suggest holding a treat and just waiting for them to get frustrated and bark. When Mud gets done running through all her tricks in an attempt to get the treat and still doesn't get it, she just gives up and will lay down and take a nap. She doesn't bark at a knock at the door or the ringing of the doorbell. She doesn't bark when you won't give her her favorite toy. She just doesn't bark. So...teaching speak is pretty low on my priority list anyway, lol.
With any behavior, just pay close attention to finding something that sets off the behavior. If they just do it randomly, then you're going to have an extremely hard time. Good luck!