**Select Quickly**

First thing to do is remember how to change what you have selected quickly, so that you don't spend any extra time trying to get a box around just the right guys to set to your group. You might already know all of these but:

 - Ctrl-Click = Select all units of the same type
 - Double-Click = Select all units of the same type
 - Shift-Box = Add all boxed units to the current group
 - Shift-Click = Add or remove the clicked unit from the current group
 - Shift-Num = Adds selected unit to control group dictated by num

Shift-Click and Ctrl-Click work on portraits too. And these work together: "Remove all units of that type from my group" is Shift-Ctrl-Portrait Click.

**Group By Role**

You will always want to group by units that you will be giving the same orders to, so you don't have to alter the group any after you select it. I find my groups work out like this:

 - Attack that way
 - Kill that scary ground unit
 - Kill that scary air unit
 - Detectors

I sometimes group by move speed as well, because sometimes you just need any troops here right now!

**Almost is good enough**

Finally, remember that its ok to miss a couple guys because new guys were built, just go pick them up immediately after. (I find Shift-Box to be the most useful for adding the new guys to the group.) All the guys in a group won't arrive at the same time anyway, and having MOST of the group start moving toward where they are needed is better than none. Just update your groups before you move away from your base, or whenever you give them new orders, but AFTER the order.