Your problem is that you're taking another program's workflow and attempting to force it onto Motion. From the beginning, what you're asking is very simple. Just follow these steps from now on:
1) Place the layer you wish to mask inside a group. By itself, if you only wish to mask that particular layer. This can be done simply by selecting the layer in the layers pane, right-clicking and choosing Group from the menu.
2) Apply any mask to the group. If you select the group, you can create bezier masks, spline masks, or even image masks for the group. Because the group contains only one layer, the effect will be the same as if you had applied the mask directly to the layer.
3) Adjust your layer. Because the mask is applied to the group instead of the layer, the mask will remain the same even if you scale, reposition, or otherwise adjust your initial layer.
That's all there is to it. Take a look at my tutorial on drop zones. It covers these steps in the context of the image mask. It may help you become comfortable with the basic workflow of masks in Motion.
[Quick Tip - Manipulating Drop Zones|http://www.youtube.com/watch?v=Kx4wn82jVLw]
Hope this helps
Andy
