We used RapidVisa. Its not cheap, but it made the process extremely easy for us. I think it was around 400?
Basically, the instructions give advice on top of the required forms. Things like labeling the pictures of you two together with dates, locations, names. Making sure you don't include too few or too many pictures. Making sure you include receipts/bank statements from both of you showing you were at the same place/same time. Making sure to include boarding passes for flights because a flight confirmation doesn't show you actually went there. We got our statements of intent notarized (not necessary but it showed we were together and got it done). Family letters aren't needed but helpful. Clarifying details on the statement of how you met. Just a lot of clarifying and providing details so the agent doesn't have to ask questions or ask for more.