I suggested the Daily from Day before, but to summarize it a little:
Don't click on items! If you don't know the hotkey, hover over it, read the key that is shown there, move your mouse back into the playing field and press the hotkey.
Of course this will slow you down and you will lose a few matches, but well, you never learn without losing. I personally don't like the grid hotkeys. They might help some players, but you still need to remember the position of the command. It's easier for me to remember the mnemonic hotkeys ([B]uild -> B[u]nker => [Bu]nker, [B]uild -> [B]arracks, [B]uild -> [H]atchery, [B]uild -> [N]exus, etc.)
Grouping Units by Numbers
What you can practice as Zerg (though you might lose matches) is to store half your zerglings in group 1, the other half in group 2. When you attack, let both attack from different angles.
For macro, as explained in the above Daily, keep your command center, hatchery or nexus in group 4 or 5 and instead of clicking the command center and clicking on the worker icon (or pressing the hotkey) remember to always press 4->[S]CV or 4->[S]pawn->[D]rone.
I rarely use them for something different than gathering my units near my base and sending my workers to work.
It's dangerous to use them for something else because they will execute a move command so if they encounter an enemy they will just die.
For a rush it might be nice to set the rally point into the enemy base or at least close, but as i don't rush i can't tell much about that.