I sent photocopies of letters exchanged (proper written ones which were actually posted), as well as extracts from social media conversations at significant times in the relationship - basically showing the date of first communication and latest, so that they can see how long you've actually been together. Obviously include wedding/marriage photos, also social or special occasion photos (such as Christmas, birthday celebrations or whatever) involving family members or friends. It's just to prove the relationship is genuine (and acknowledged by others), but you don't need to give a blow by blow account of everything. On one of the forms you submit you have to state when and how you met anyway, so that's sufficient. There's also space in the form to add further details supporting your application, so in that bit you can give a bit more detail about the relationship if you want.