sorry to hear about denial of visitor visa to your parents.
Please go to this link, it has information about all the documents that probably would be good to include.
http://sajha.com/sajha/html/index.cfm?threadid=108376&noofposts=9
Last week, my parents got visitor visa for my graduation.
My dad is retired and my mother is a housewife.
Few things.
1. I assume you are sponsoring your parents for visitor visa(affidavit of support (I-134) ma you must have included that you will be responsible for all their expenses while they are visiting you
in USA)
In that case i think at least some amount in your bank account would be fair to show.
May be Around 10K to 20k would be sufficient.
if you are not sponsoring your parents and they are going to cover their expenses by themselves, then I think they have to show the decent bank balance, property.
2. I assume the language for interview process is in Nepali. It is better to have in Nepali.
3. Having ticket beforehand is good idea. However, unfortunately what if again the visa is rejected. you never know. If you can reimburse the amount of ticket then it is fine.
4. Google some visitor visa interview question and answers and prepare your parents for it. i think only one person should speak during the interview.
indicating Strong tie back to Nepal so that your parents would return back after couple of months would be helpful.
strong tie like (your house, business) no one is there to take care of those for long period in the absence of your parents)
for now your relative is ready to take care for couple of months and so on.
I assume you have a letter from your university. that is one of the most important documents.
Good luck. Wish your parent get visa this time.