How do I keep the weight of my foot in the Stirrup?

My trainer is always telling me not to float my foot in the stirrup, I am having a hard time keeping it on the ball of my foot. First check your seat, are you sitting centered and balanced, an open relaxed hip, feeling your seat bones sink into the saddle. Each of your legs should work as two separate parts(thigh and lower leg). the easiest way to keep your weight in the stirrup is to allow the inside of the foot to be slightly lower than the outside.