Ah yes, I think we've all gone through this at some point and it's very frustrating. You're going to need to make sure your light source is very stable and does not change between frames. Block out any natural light and make sure you do not have an inconsistent or flickering lamp. You should also check your camera settings and make sure nothing at all is set to auto adjust, e.g. white balance or exposure.

Also, there is something I do when masking a shot like this that I will mention. To keep the shadow consistent, I first take the original frame and mask out the support. Then I take another frame dangling the figure from overhead and casting the shadow in roughly the same place and I mask this shadow into where there was a gap in the previously masked frame, with liberal use of the blur and smudge tools in GIMP to make them blend. My final result looks like this.

I hope this helped!