what the others said + other examples could be:
- sms logs from ur mobile phones, phone bills
- gift receipts for each other, receipts from outings u had together, ticket stubs from concert, movie, etc
- testimonials from friends/families - they should b from both sides (these r not required for married couples but they don't hurt; on the other hand for common-law they r required and they have to be notarized)
- engagement pictures, party, invitations
- relationship history (essay) from both u and ur sponsor
read old posts from here (search for love_young and her toc) and also the manuals Allison said and take ur time!!!

good luck!