I have been paying IRS monthly installments for nearly four years. Anyway to negotiate down now?

I have been paying off taxes by monthly installments, as a result of threat to garnish my salary. Each month my bank account gets hit with a sizeable payment. I still have about $20k to pay, including penalties and interest. Anyway to now negotiate, such as waiving penalty and interest, or principal even? Should I go to a tax reduction consultant or would I just get ripped off?