Make sure that subject and verb agree (either singular or plural). This can be tricky to spot, especially if we look to the part of the subject sitting closest to the verb to inform our decision. Here is a very simplistic example: The central tenets of the argument are as follows. NOT: The central tenets of the argument is as follows. You can apply this principle logically to longer and more complex sentences.
Remember which style you are following. For example: I am going to analyze the organization’s stats [US style]. Or: I am going to analyse the organisation’s stats [UK style, -ise suffixes]. Or even: I am going to analyse the organization’s stats [UK style, -ize suffixes]. But not: I am going to analyze the organisation’s stats [mixture of US and UK style]. Also, consistently use either single or double quote marks (and the opposite for quotes within quotes). There are many more examples but these are the two most common that I work on.
Keep things simple and avoid repeating yourself. For example: The dog remains the most popular pet. Or: The dog is still the most popular pet. Rather than: The dog still remains the most popular pet. [Still and remains do the same job here.] This is a very basic example, but I try to follow the maxim of ‘less is more’ in writing.
Use semi-colons and colons correctly. For example: The cat sat on the mat; the mat was red. [A comma is not sufficient to separate these two parts of the sentence.] Or: The cat has some issues: wariness of strangers; greediness with food; and being a fidget. [The first part of the sentence introduces a list using a colon, and semi-colons separate the items in the list.]
Certain words are easy to mix up, especially for non-native speakers. One of the most common words that I correct is costumers, which is a conflation of consumers and customers. Incidentally, someone who makes costumes is a costumier.
Comprise stands alone. For example: The business comprises five units. NOT: The business is comprised of five units. You can, however, say: The business is composed of five units.