' ~ ' is also very handy, as it selects ALL troopers. (By default, '1' is row 1, and '2' is row 2, etc). Then you can just press that number any time while playing, to select those troopers again. Just use CTRL and Click to select some troopers, then hold ALT and press a number key to save that selection set.
I also like to be able to save custom selections for groups of troopers. S - Stop (stops all troopers from doing whatever they're doing) F - Fire Artillery (then click on target to fire there)