1. Letter of your employment.
2. Bank Statement - couple of them
3. Affidavit of Support-Notarized
4. A personal invitation, inviting your parents abroad, addressed to the Consulate officer.
5. Your Visa Receipt Copies
6. Copy of your passport,I94's etc.
7. Paystubs{Just in case,not needed, but better to include}
[If your parents can show some proof of some property/father's business/employment in India etc, to convince the visa issuing authority that they would be returning back, that would be great..!! - Not a Necessity though]
Don't remember off-hand of other documents, but these are the ones I remember that I sent 2 yrs back for my parents visa.
Hope this helps
Good Luck
PS: Do a search and u might get some more info..