Asylee13,
You do not need much vor a visitor visa for your parents. As far as I know, all you need to send is an affidavit of support. (It has guidelines and income requirements, though)
The most important thing for your parents, to be able to get a visa, is that they can show to the American consulate or embassy, that they have every intention of returning to their country after the visit, and not seek any immigration benefits here.
They need to show either proof of income or steady job or business, land or property ownership, or other children and family toward whom they may have responsibilities in their home country, which would require their prompt return. It will also depend greatly on your parents' age.
Toward some countries they are more lenient and toward others they have more stringent requirements; meaning if you are from a certain country it is easier to get a visitor visa to the US, than from others.
I was able to secure a visa for my aunt through my congresswoman. I sent my aunt an invitation letter for my graduation, and gave a copy to my congressoffice; they sent a letter to the counsulate in the country where my aunt was applying.
Good luck,
AR
P.S. My aunt was granted a visa, but my young male cousin was not.
