Need help figuring out a formula to add a carry over balance if there is one

Hello, and sorry I'm new, and it has been quite a long time sense I've used excel. I have put together a simple program to keep track of what people owe each month to a club. I have 12 columns (one for each month) and like 10 rows labeled with the general things that they get charged for membership fees etc. So I summed up column B =Sum(B2:B29), for the total due that month, below I have a paid line B32, and below that a Balance line =B30-B32. In the next column (month) I summed column C and added the balance from Column B =SUM(C2:C29)+B34. The problem comes because I used that formula it carries over for every month. if January has a $20 balance, it carries over all the way until December until it's paid and I'd like to have it so that only months with a balance show it. Example Jan $20 balance, Feb $10, January would carry over into February, but not into March until I've entered an amount for March. I'm sure that there's a way to do it, my brain has just turned to MUSH at this point and can't think of it!! Thanks for you help!!