Lean on a Tenant to Pay Up

[Amy E.]

If you have a tenant who is paid up on rent, but doesn't pay for other fees (storage, parking, late fees, whatever), how hard do you go after them?  I'm in a situation like this and the property is profitable, and the rent is paid on time but the tenant essentially owes me $325 for back payment on a storage unit with the potential to get even further behind.  Its not part of their rent, it was an add-on.  The storage unit is on the same property as the rental unit and its empty.  But its in their lease that they will pay the extra fee for a year.

I can't seize the property in the storage unit, because its empty.  I could go to court, but that feels like a waste of my time and effort for a property that is already profitable.  I do not have a very commanding presence, and while I talk sternly to tenants if required, I feel like I have no leverage in this situation because the tenants will keep ignoring me unless I "do" something, not just talk.

Making matters worse is the fact that they are dirt poor, and my sympathy meter kicks in.  I have to beat it back a lot and remember that this is a business.

 Thoughts? Or is there a method for leaning on people to cough up the cash they owe without having to take legal action?