If your feet are getting lighter in your stirrups, you are pulling your leg up in an attempt to grip. If you notice that, relax the leg, imagine there is a waterfall flowing down the back of your leg. When you grip and tense, it stops the waterfall. Let the water flow down out of your heel.
Something else you can do with no stirrups, draw the alphabet with your toes. You can't grip with the leg when your toes are moving. Start at the walk eventually the trot. Its a weird feeling at first because you'll be so used to holding yourself on.