I have sponsored my parents too and I have submitted the following documents so I recommend you for preparing these docs:
1. Affidavit letter I-139
2. Bank Balance letter
3. Your status and employer job verification letter
4. tax return 2006
5. Invitation letter from you to yr parent
6. Must ( bank balance letter and property evaluation from yr parent)
7. Better to apply both of yr parents (I applied for my dad first was rejected and when applied both ad visa issued)
8. visa apps from nepal to US EMBASSY

Better to submit good bank balance letter from yr parent. US Embassy ask strong balance letter from yr parents as if happened us us.