It's been a long time since I last posted, been too busy with school to play, but I wanted to let you guys in on another training concept that you might find useful.
Use every single one of your hotkeys from the beginning of the game to the end. You've only got an army of four marines at the start? Then hotkey '1' for all four, '2' for three of them, '3' for two of them, '4' for one of them, '5' for the other one of them, 67890 for the buildings.
Train yourself to utilize every hotkey, even when it is inconsequential such as at the start. When you need to use them all late game you won't struggle to move between them. Your micro at the start will improve as a result of the intelligent hotkeying and your general speed will increase as you have to reset keys whenever new units are produced. It is through this method that the pros control their units so well (being able to split them apart quickly, circle around chasing opponents, surround with weaker units, pull back injured ones) and manage large scale battles.