Watch for proper subject-verb agreement. This may seem obvious, but beyond the basics, such as “Writing skill IS important” vs. “Writing skill and grammar ARE important”, it isn’t always so clear.
This is often the case if there’s a prepositional phrase somewhere between the subject and verb. It’s really easy, and I’ve done this a lot myself, to take the subject of the prepositional phrase as the subject for the verb. I’ve even seen such things slip past editors from time to time. For example, “The mother of the kittens IS grey and white”. To get rid of the troublesome prepositional phrase you could say “The kittens’s mother is grey and white.”
I think everyone hated diagramming sentences, but this is a good example where having those skills helps. Too many people think because they can talk that they can write. Thus, they “may be talking as they be speaking”. Correct grammar is important if you want to build a solid reputation as a professional writer.