While this isn't really a RTS, there's no harm in stealing the most important control mechanism from them - group hotkeys.

Press CTRL - 1 and the "thing" is saved as group 1.
Press "1" and you quick select that same group again. Double tap for zoom to. You all know how that works. =P

So what "things" should be saved?  (in this order)

1. primarily the leftmost unit that is highlighted in a city or army display, accompanied by the other units that are highlighted there
   Armies are completely temporary constructs that always dissolve without a trace when entering a city so it would be pointless to save "an army".

2. City, if a city is selected.

3. If an army outside a city is selected:
    primarily the leftmost unit in that army, accompanied by the other units.

4. If nothing is selected:
    the spot on the map.

For recalling a group:

The primary unit is the one that matters here. Accompanying units would only be selected/highlighted if in the same location and if previously saved in the group selection..
If the former leader dies, the #2 could of course step up and become the selection's effective leader.

This would not only help with army management, it would also help with the jittery mouse issue because you would not have to go through the clickeroo everytime your main army stack was to leave the freshly conquered city.

And last but not least... it's the obvious thing to do when pushing groups/armies around. When I had 2 or 3 units in my first stack I automatically hit Ctrl-1. Pressing "1" later, the camera zoomed... somewhere. Cool. But where was my unit? It wasn't anywhere close... what's going on here?