Averages that exclude 0 values

[shannaj via AccessMonster.com]

I have a database that is tracking customer sales for the past two years. I
am trying to find the monthly average for 2006 and 2007. The problem is that
I would like to exclude the 0 values. Especially with this year's average
because the year is not complete, so I have 0's showing for Sept. Oct. Nov.
and Dec. I know I can avoid some of the problem for this year's figures by
only dividing the sum by 8 instead of 12, but if there is a way where I do
not have to go in every month and chang numbers, it would be great. Can
someone please provide some input?

Here is what I have for 2006.
Avg Monthly Sales 2006: (nz([Sum Of qryCustomerSales2006CT_Jan]+[Sum Of
qryCustomerSales2006CT_Feb]+[Sum Of qryCustomerSales2006CT_Mar]+[Sum Of
qryCustomerSales2006CT_Apr]+[Sum Of qryCustomerSales2006CT_May]+[Sum Of
qryCustomerSales2006CT_Jun]+[Sum Of qryCustomerSales2006CT_Jul]+[Sum Of
qryCustomerSales2006CT_Aug]+[Sum Of qryCustomerSales2006CT_Sep]+[Sum Of
qryCustomerSales2006CT_Oct]+[Sum Of qryCustomerSales2006CT_Nov]+[Sum Of
qryCustomerSales2006CT_Dec])/12)