Tipster Ed Gerl advises our readers to lock their viewports. It is a good habit to get into when you make your viewport in paper space.
"Create your viewport, set the scale, and position. Select the viewport and lock it. I've been locking viewports since version 12; once you get into the habit, it just comes automatically."
Notes from Cadalyst Tip Patrol: I couldn't agree more. Lock your viewports — all of them, every time! It is easy to do and there are several ways you can lock them. Whichever way you choose, start by selecting the viewport. On the status bar there is an icon of a lock; if it's open, then the selected viewport is not locked. Click the icon to lock it. You can also lock it this way if that viewport is active. Another way to lock it is through the Properties Manager palette: Select the viewport, scroll to the Display Locked field, and click Yes.