I've been doing a lot of rotoscoping recently. One thing I found myself doing constantly was locking the position property of the layer using expressions. By using expressions, I can be sure the layer isn't moving anywhere, but I am still able to work with it, unlike using the layer's 'Lock' switch. By locking the layer before I start rotoscoping, I can make sure the layer iteslf does not get shifted while moving around mask points. There's nothing worse then finishing a big chunch of roto only to realize that at some point you had moved the layer instead of a mask point. All of your keyframes would then be wrong for as long as the layer has been shifted.
Now honestly, all that it takes to fix this problem is adding a very simple expression to the layer properties you want to lock. basically you just want to take the layers current position and set it explicitly inside its expression. So if your layer's position was 960, 540, you would simple put the expression [960, 540] into the position attribute. Now even though this is pretty simple to do, its something that I found myself doing over and over again and decided I could save myself a little time by writing a script to do it.
Just drop the Quick Lock script into your SriptUI folder and you can open the panel from the Window menu. To use it, just select the layer you want to lock, then click the button for the particular property. The button labels will update accordingly (between lock and unlock) depending on the status of the layer. If you are using After Effects CS3, the labels may not update correctly but the functionality will still be there.