Get really good at campaign tagging: Even amongst data-driven marketers I encounter the belief that UTM begins and ends with switching on automatic tagging in your email marketing software. Others go to the other extreme, doing silly things like tagging internal links. Control what you can, and your ability to carry out meaningful attribution will markedly improve.
Beyond organic and direct traffic, you must understand the difference between all of your traffic sources and how traffic is classified. Most web analytics platforms, like Google Analytics, utilize an algorithm and flow chart based on the referring website or parameters set within the URL that determine the source of traffic. Here is a breakdown of all sources: