While writing the tip on Removing Numbers from String, I encountered this problem. My formula was too long to be contained on the page. Hence, I had to span it in many rows.
To do this, you just need to press ALT+ENTER at the place where you want to continue the formula to a new line.
From my experience, I have noticed that if you try to break immediately after a function and before "(" bracket, Excel gives error. Hence, you can't break SUM(A1,B1) into
SUM
(A1,B1)
You can break at any other place though.
Also, you can't break the function itself. Hence, you can't break SUM(A1,B1) into
SU
M(A1,B1)
Credits
1. http://excel.tips.net/T003043_Dealing_with_Long_Formulas.html