Anything that is described in the guideline and can help prove that your relationship is genuine. Phone bills ( if you are apart), chat logs, picture of the two of you together with family and friends, letter from friends and family, facebook updates if you have any proving that your friends are aware that you are a couple, plane tickets and copy of visa if you went on holidays together, if you visited him or he visited you, etc.... anything that can help convince the VO that you are a real couple. If you live together, you might get mail at the same address - you can also send that (or a copy of it if you don't want to loose it).
good luck,
Sweden
