When gathering your evidence, it helps to keep in mind the 'four-fold test' that the assessing Immigration Officer is going to be looking at your relationship against:
'Credibility': any statements made and evidence provided needs to be credible (ie. believable to someone who knows nothing about you)
'Living together': the evidence needs to show that you live together at the same address
'Genuine partnership': the reasons both partners are in the relationship are genuine (ie. there's no inkling that the relationship is "set up" just for the purpose of the visa application) and both partners intend to stay in the relationship long-term, without starting relationships with others
'Stable partnership': evidence shows that the relationship is likely to endure (ie. not break up as soon as the visa is granted)