How do you get an if statement to do nothing?

I have 2 spreadsheets that get updated monthly. one is overwritten, and I
then want the new data to automatically insert into the column for the
corresponding month in the other sheet.

=IF([Asset Allocation.xls]Asset Allocation'!$C$5=$Z$2,[Asset
Allocation.xls]Asset Allocation'!D8,0)

So if C5 in the other sheet is the same as z2 in this sheet, put in the data
from D8 into this cell.

Problem is, when you move to the next month, the statement is no longer
true, so it inserts a 0 or blank string. How do I get it to do nothing??
(thereby leaving the existing data)

Is there an easier way of doing this??

Thank you