u may need following:
-copy of ur I-20
-ur invitation letter
-invitation letter from ur school (usually ur int'l adm. officer writes one)
-proof from school stating that u're graduating in December
-if u have apt., send the copy of the lease docs, so ur parents can say that they r gonna stay at ur apt.
-the duration of their intended stay must not be more than 5 weeks.
-ur parents must be able to prove "non-immigrant intent" that means they should be able to furnish documents that will force them to return Nepal after the graduation ceremony.. the docs that will fulfill such criteria would be: ur parents business in Nepal (if they have), their jobs (if governmental, is best), if u have siblings in Nepal that may also be another reason ur parents could return, if ur parents r pension holder from their govt. job that can also help. So, main point is "ur parents must be able to prove to the consular of their 'non-immigrant intent', by showing various things that shows their ties to their home country." If u've siblings in US, do not hide that fact, coz the consular knows everything. Ask ur parents to be truthful as much as they can, and just don't lie. That will be it for increasing the chances of getting the visa, rest is up on the interviewer and interviewee, and all u can do after that is PRAY! well, it's getting late now for December, so better hurry up & Good Luck!