If you are ineligible to adjust status to that of a lawful permanent resident due to certain grounds of inadmissibility, you may be able to apply for a waiver based on hardship. Waivers are decided on a case by cases basis. You should seek the advice of an immigration attorney to determine whether or not you qualify to file a waiver and the strength of your case. An immigration attorney can also guide you through the process and help you determine what types of documentation you need to support your case.
Furthermore, there will be a new stateside waiver program available for certain immediate relatives of United States Citizens who are inadmissible to the United States due to the accrual of unlawful presence. You must be age seventeen or older in order to file and you must have an approved I-130 relative petition. These waivers will be able to be filed in the United States before you have your interview abroad. This process will take effect on March 4, 2013. You will not be able to file these waivers before that date. This process is still very new. As such, you should seek the advice of an immigration attorney to learn more about the process.