Security Deposit Increase Clause in Lease

[Kay March]

I normally require a security deposit equal to one month's rent plus $200, and I raise the rent annually by $25 per month. For tenants who stay year after year, the security deposit over time shrinks in relation to the monthly rent to the point where it's clearly inadequate. Does anyone have a clause in their lease that provides for regular additions to the security deposit to prevent this problem?

And for current tenants whose lease does not contain such a clause, can I just demand that they add to their security deposit at some point?