If you're actually exporting maps in different projections, as your first sentence says, the maps probably won't line up even at the same scale. Different projections have different distortions of shape, area, distance, and direction. Sometimes it's subtle, and sometimes it's extreme.
If you're actually using the same projection and just want to create an identical map series, there are a few possible solutions. As PolyGeo mentions, Spatial Bookmarks are a good way to mark a particular view to return to it later. A manual way to do this (without making a bookmark) is to select a feature at or near the center of your desired view and use the Zoom To Selected option (right click the layer). You can then type in the desired scale in the scale dropdown box (or select it once you've typed it the first time) to 'zoom out', and this should get you a consistent view each time.
The Dataframe also has a property that can help - Extent. The options are Automatic, Fixed Scale, and Fixed Extent. With Fixed Scale, the zoom should be right even if you change the size of the dataframe - though the positioning within the dataframe might be off a bit (note you can always type in or select a specific scale in the scale dropdown box to get back to the same 'zoom level' - you don't have to fix the scale at that level). With Fixed Extent, the dataframe will always show the same area no matter the size, but it will change the scale if you alter the dataframe size.
My workflow for a situation like this is to create a template .mxd file. It has all the elements and symbology that will be common to all of the maps - both in the map and in the layout, such as titles, north arrow, scalebar, etc. There are then two approaches to get the individual maps:
add everything from every map and then save copies of the
template, turning off or removing unused layers from the copy (layer
groups might come in handy for this)
save .lyr files of the layer symbology unique to each map (either groups
or individual layers) and add those to a saved copy of the base
Another tip I would give is to remember that .mxds are just displays and symbology of data - they point to the data but don't contain it. This means if you want to avoid messing up your view/scale in your print map, you can do all of your editing in a different .mxd file that doesn't even have a layout set up in it. Once you've done all your editing and saved the changes, if you open up your proper map layout all the changes should be reflected there as well (again in terms of data, not symbology), but the view and scale shouldn't change at all.