It isn't illegal to keep on doing under 10k, it's illegal to hide income from the gov't no matter the amount. In order to prevent ppl from hiding money, the IRS requires banks to file if someone deposits 10k or more. Since there isn't any income being hidden, there is nothing illegal being done (insofar as tax evasion goes). It is unlikely that a single deposit of >10k would trigger an audit of a person who files taxes, but most ppl don't feel the need to test Uncle Sam.
I happen to know an IRS agent, and what he told me was that multiple deposits of slightly less than 10k (wouldn't say how much) can also trigger an investigation. Banks are required to report all "suspicious activity" to the IRS. So you guys may want to dial it down a little more if you are doing it a few times a week.