There are a couple of situations that may apply to you that the IRS will allow leniency in collecting your back taxes.
First, if you are experiencing any significant financial hardship, that causes you to be unable to currently pay any of your back taxes, notify the IRS. A significant financial hardship example would be loss of job, disability, etc. The IRS may be able to temporarily suspend any collection efforts on your account. Just explain to them what your significant financial hardship is and why you cannot pay your back taxes.
Even by temporarily suspending collection efforts, however, interest and penalties will still be accruing on your back taxes.
Also, if your or your spouse are a member of the Armed Forces, you may be able to defer payments of income taxes. Contact the IRS for special circumstance criteria.