If you mean that you have multiple 'worksheets' (many tabs in one file), then all you need to do is go to your year-end tab and set a cell equal to the value of the monthly total for month x, example formula in the year end summary: (=Sheet1'!$C$20.) Repeat for each month so that you have a series (of 12?) cells in the yearly total sheet that each return totals from each month sheet.
Alternatively, you can also use separate Excel files for each month, and then use a separate file for the yearly totals. You'd set this up much the same way, but instead of referencing totals from other sheets in the same file, you'd need to reference cells in another file.
What I would suggest is that you create a single Excel file designed for any given month. Use this as a template, so that when you start a new month, you just 'save as' the template as 'June 2007' or something, and that way you won't have to deal with tons of tabs in one huge file over time.
Then, you have a yearly total template where the monthly totals are easily referenced by setting a formula to (='[June 2007.xls]Sheet1'!$C$20) or whatever the total cell is (C20). Alternatively, in the monthly template file, you can name the total by going to insert->name->define, and then you select the total cell, name it 'total' or something. Then your formula in the yearly summary is (='[June 2007.xls]total). Using names can make it easier to modify complex formulas that reference crazy crap that you don't want to keep track of.
The only trouble with using multiple files to contain the data is that you can't go moving the files around too much (as in changing the directories) or you'll break your links. Also, in order to make sure all current data is being referenced, you may need to re-open files - this will only be a problem if you have both a month file and the yearly total file open at the same time - if you update something in the month file, you'll need to save the month file, then close and re-open the yearly total file to see the updated info.
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