You need the following:
1. Copy of your I-20
2. Copy of your passport / visa
3. Copy of your I-94
4. Your transcript
5. If you want to support financially (for their travel expense, room and board) then fill out I-134 form.
6. Your bank statement
7. Copy of your apartment lease (if you say that they are staying with you) (I think this is optional, but I did it)
8. A letter to your parents inviting them (This is just a formality)
9.  A cover letter to the consular to grant your parents visitor visa.

These are the documents that I sent.
Your dad needs to show strong ties to Nepal such as:
1. bank balance
2. Property
3. Letter from his job (if employed) or letter showing he is receiving pension (if applicable).

Hope that helps

Cheers

Bravo sparty

 You can send any documents from here  like your apartment lease, your vehicle registration card, a request letter to the consular blah blah blah..................Your parents are applying for a visa to attend your graduation so I dont think its a good idea to show your bank statment and all other craps. First of all you havent graduated yet and you already have all kinds of money in your account. I would think about that. Here is what i did and the basic documents needed to apply for the visa for graduation for your parents:
                             - Your I-20 (updated and signed)
                             - Your Passport/Visa/I-94 photo copies
                             - Invitational letter from school
                             - Financial documents from Nepal (source of income,bank balance, property evaluation)
                             - you can add some documents to reflect your good academic standing (transcript, any accolades you won)

These are the basic documents that a parent need to submit. Anything besides these are just a way to console and convince your self. Good luck!!!