I do this so seldom I always forget how to do it. Personally I really don't advise doing this as it's just a nasty thing to do and can cause an incorrect interpretation. Spreadsheets should endeavour to be easy on the eye but not at a cost of functionality.
However when one is asked to do it by higher powers one must obey.
Ignore idiots that fill every cell if stupid formulas like =if(A1 = 0,"")
Just do as commanded on this article here. http://office.microsoft.com/en-us/excel-help/display-or-hide-zero-values-HP010070461.aspx
This is for 2007 - 2010. For 2003 the same thing is under Tools->Options... or something like that. I haven't used 2003 for years.