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Workflow threshold from Rollup sends too many e-mails

I want a e-mail to go to a customer when an Account rollup field (rolled up from Opportunity) reaches a certain threshold. However, I want the e-mail to go out only once per month.
I set up a workflow rule to trigger when the threshold is reached, however, it triggers every time the rollup is changed.
I figured I'd create another field (e.g. 'E-mail Sent' checkbox), and set it when the workflow is fired. However, I can't figure out how to reset the checkbox automatically on the first of the month (workflows only work when something is edited).
Anyone have any ideas? Work-arounds?
Thanks very much.