Although we don't need advanced degrees to be writers, we do need to keep our skills sharp to stay on top of the slush pile. While many writers have attended college, have their MFA or English degrees, there are just as many who have earned their degrees from the school of hard knocks. What all of us can do is take classes, participate in critique groups, and be active on forums. Many of these activities are no cost to us, but offer useful information and ongoing education.
Currently, I am participating in a workshop offered by Forward Motion (http://www.fmwriters.com/). In order to participate, you have to register, but classes are free. The workshop I'm involved with is "Non-Verbal Communication." Although I've been writing, and publishing, for a number of years, I am learning a great deal through this workshop. In the early exercises, I thought, "I know this," but it soon became apparent, I was mistaken. Margaret Fisk is the moderator and instructor for this workshop. She offers insightful criticism and encouragement. All attendees are encouraged to comment on each others' posts.
There are a number of writing classes available on-line through community colleges, writing groups, and those offered by writers through their own newsletters. Check them out. Ask for advice from other writers. Sometimes free classes aren't worth your time. Others, like those offered by Forward Motion, are well worth your effort. While you may think you understand the mechanics of good writing, you can always fine tune your abilities.