Balance question

Okay, so when Gaia was here, she pointed out that one of the reasons that I was becoming unbalanced in my seat was my tendency to draw my knees up and cling to the horse. I've been practicing putting my weight into my heels and was wondering if you all had any suggestions to guide me - I consistently forget about it and find that my heels are down with no weight in them at all... or am I misunderstanding the whole thing?

Any advice is appreciated!