Cropping question

I read memories 'til my eyes crossed, but I didn't see my question adressed. When you are cropping screencaps for an IR animation, are there any tricks you use to make sure you get the same thing in every shot? If you are off a bit, it gives the animationa a jumpy look, as though the camera moved a foot or two. I find it hard to get exactly the smoothness I want even when I use a fixed proportion cropping tool. How do you line it up? Use the blue guides? Any feedback or pointing to tutorials appreciated.




I use PS6, though I don't think it matters to this question.