Freezing and falling over

Does anyone have tips on how to control the freezing episodes.  I get symptom several times a day.

 

When my meds where off or even before I am due to take more pills, Suddenly feel like my feet are glued to the ground and feel as though I am going to fall. When I try to make a move I shake and do fall sometimes if there isn't any thing to grab onto. When I really conncentrate I can manage to move again. I have had PD for 15 years and this is by far my most difficult symptom.