F4 to Repeat the Last Action (forget about Ctrl + Y)
I had been using Excel for decades before I found about this tip.
When you want to repeat the last action that you have just taken, simply select the next cell or range where you want to repeat it and press the F4 key.
I have found myself using this most often for quickly adding and/or deleting rows or formatting a cell with a particular color (especially useful if I want to highlight cells where I have hardcoded a number or a column reference).
For example, to insert a row with this method, insert a row as you normally would using the mouse. Then, move to the active cell where you would like another row inserted and press F4.
Rinse and repeat as necessary.