If the petitoning sponsor (e.g., the USC spouse) does not make a sufficient income or doesn't have sufficient assets, then the problem can be resolved by having a co-sponsor or joint sponsor 'step in the shoes" of the petitioning sponsor. A joint sponsor must be 18 and a legal permanent resident or a US citizen and must be domiciled (residing in the US). The joint sponsor's income is based on his/her "household size." This includes generally spouse, children under 18 and any other dependents on last tax return. You can find the threshold income he/she is supposed to make for that size household on Form I-864P (www.uscis.gov - go to forms section).
The petitioning sponsor must still execute an I-864 and show most recent tax return/W2/1099, even though do not make enough income. If did not file a tax return, must explain in a letter why not required to file a tax return.
Please note that the immigrant spouse, if making legal income, can file an I-864A and his/her money can be counted in the petitioning sponsor's "pot" to make the income requirement. If obtained income illegally, then cannot count.