Once you have created sheets, its nice to work in the frame of that sheet, but Revit is really set up to use views instead. A nice alternative is to open your sheet, right click on the view and pick Activate View.
Now you can design directly in that view within the context of the sheet. This can really help when flag notes are being used. To deactivate the view, simply right click on the view and select... you guessed it Deactivate View.