Excel - reset the last cell on a worksheet

• To select all columns to the right of the last column that contains data, click the first column heading, press <ctrl><shift><right>
• To select all rows below the last row that contains data, click the first row heading, press <ctrl><shift><down>
• On the Home tab, in the Editing group, click the arrow next to the Clear button, and then click Clear All
• Click the Microsoft Office Button , and then click Save
• Click the Microsoft Office Button , and then click Close
• When you open the workbook again, the last cell of the data should be the last cell on the worksheet

Excel on Mac/Windows - linebreak in cell

On Mac - control-option-return
 
On Windows - alt-return (or alt-enter)