Copy of PP and Naturalization Certificate
Taxes (although only the last year is required...going back 3 years is not a bad idea).
Recent Job Letter and last three pay stubs
The Beneficiary (applicant) should submit
Copy of PP
All past immigration forms/visas
Bona fides that should be submitted by both:
Pictures of the wedding.
Pictures of vacation together or family gatherings.
Make sure driver's license are updated to show same address.
Any documents showing that one is the beneficiary of the other in case of death etc... (life insurance, wills etc....)