I agree that it's best to use an accounting program to generate invoices. That said, maybe a minor tweak can solve your problem in Excel without having to start over. I'm assuming you don't already have a system of storing invoices in a monthly file.

Do your individual tabs have any formulas on them that refer back to another sheet in the spreadsheet? If not, it would be very easy to move a month's worth of invoices to another spreadsheet.

If all the tabs for the months's invoices are beside each other:

1. click on the first tab, then hold the "shift" key down, then click on the last tab,(you can use the mouse to scroll across the tabs if you need to).
2. Now, right click on one of the selected tabs and choose "move or copy".
3. In the "To book" section, pick "(new book)" from the drop down list.
4. Click Okay.

You have now created a new spreadsheet with all the tabs that you had selected. You need to save this file right now. I would use a file name that is meaningful and sorts easily in a file management window. Eg. I would use "200903 invoices.xls" for the March 2009 invoices.

If you do have formulas that refer to some other sheet on your master spreadsheet, there is another step you need to do. If this system makes sense to you and you have that problem, I can easily walk you through it.

Again, an accounting program would be better, but there could be a big startup commitment to get going that you might not want to do in the middle of the busy season. My method of handling your current invoices might allow you to make the accounting changes at your own convenience.