1. I-797a for status change. I changed to F-1 via school office, and local INS office stamped directly on my original I-94 card, so I did not have one, my lawyer PJ decorated this in the cover letter.
2. Evidence for on-compus work. I worked for XX Research Labs to sponsor my study (as a RA), and INS said they could not find the ties between the school and the employer. I submited W-2s, pay stabs, and school personal action form(all title with school).
3. ALL the I-20s. I sent email to school and requested FAX all my I-20s, they faxed all the school-copy to me(I only has the last one -student copy). Note, if you have dependents on your I-20s, you probably need to state clearly where it showed this info, to avoid another possible RFE, since the school copy is slightly different from student copy.
Also, the most common RFEs are: Medical (esp Xray), and Employment Verification Letter.
Keep these handly, in case you need them in a hurry but could not catch the person to help.
Best Luck,