Avoid telling your students that any grammar structure is a hard-and-fast rule. English is always evolving, and native speakers may use any given structure very differently, depending on location. Instead, instruct students to focus on grammatical patterns as a measure of predictability and be open to changes to that pattern. For example, it's common to pluralize nouns simply by adding an 's' to the end of the word, but this is not the only way to do so. Because you cannot teach every method of pluralizing at once, teach the most common pattern and allow students to discover others.
While most native English speakers know the rules of their language, they frequently ignore them in casual speech. Teach your students this and share a few common examples. For instance, while it's correct to say, "He doesn't know," it's also common for native speakers to incorrectly say, "He don't know." ESL students, particularly those with low levels of confidence in their own ability, can become frustrated when they encounter this. Prepare students for grammar misuse and encourage them to ask questions when they hear it.
Encourage your students to step away from the textbook and explore grammar points on their own. Textbooks frequently teach only the most basic form of any grammar structure, often omitting clauses and exceptions that complicate the sentence but also help tremendously in learning. Help students expand and solidify their understanding of any grammar structure with non-ESL literature and exercises that force them to use their current knowledge to decipher more complicated versions of the structure. For example, instead of the bland "I have gone home," let students tackle "If someone calls, tell them I've headed home for the day."
Even correct native use of English grammar can be maddeningly difficult for many ESL students. It's extremely common and accepted for native speakers to manipulate the sound of grammar structures to suit the pace of their voices or the tone of the situation (formal or casual). For example, "I would have gone" tends to morph into "I woulda gone." "You had better leave" becomes "You better leave." Teach students that native manipulation is not necessarily incorrect, but it can be very different from what a textbook teaches.
Speaking with perfect grammatical precision should not be any ESL student's goal. Fluency, or the ability to meaningfully communicate an idea to a native speaker, is far more important than mastering every grammar rule in the book. Mistakes that do not change the meaning of a sentence do not necessarily need to be corrected. For example, if you ask a student why she is late and she responds, "I have missed my bus," her level of fluency is fine. Time and practice will often eliminate these mistakes without your interference.