If you have an element which you need to modify the margin, padding, width or height for, you can use the cursor keys to increment/decrement the size. Simply use the up and down cursor keys to increment/decrement by a unit of 1.

You can increment/decrement by a unit of 10 by holding the "Shift" key whilst pressing the up and down cursor keys; and you can increment/decrement by a unit of 0.1 hold the "Alt" key whilst pressing the up and down cursor keys.

You can also increment/decrement by a unit of 100 by holding down the "Shift" key whilst pressing Page-Up and Page-Down.

These shortcuts are especially helpful when you are unsure of exactly what the correct size should be.