Keep your goals simple and straightforward when planning for class. One or two students will inevitably lag behind, or you may overestimate your students' abilities and fall far short of your goal. Plan optimistically, but cautiously. For example, rather than set the goal, "Students will demonstrate mastery of future passive tense by writing a 10-sentence story about the world of the future," settle for, "Students will be able to recognize passive tense in text." Recognition is simple; mastery is not. Don't set the bar too low, but don't frustrate your students, and yourself, with complicated goals.
Ask yourself, when planning, "How likely is it that students will see immediate application for this skill?" If you can't honestly answer "Very likely," reconsider your goal. Students want to use what they've learned right away in the "real world." It may be fun for you to teach, and hilarious for your students to learn, about idioms, but the goal "Students will correctly use five idioms" is a bad goal because it's irrelevant for most ESL students (except extremely advanced). Focus on setting goals that arm students with knowledge and basics that they need and can use right now.
Knowing your students is imperative for setting realistic goals. You'll always have strong and weak students; aim for the middle so that the weak students are challenged and the strong students are engaged, but neither group is bored. If you have a group of beginners, for example, a good goal might be, "Students will be able to have a short introductory conversation." You can easily teach simple greetings, "How are you?" and basic responses by putting a short script on the board and modeling it with your assistant or another student. Strong students can adlib if they like and weak students can challenge themselves by not looking at the textbook or blackboard when speaking.
Never set goals that you can't quickly and easily measure. You need to see whether your methods were successful and students need to receive immediate feedback. If you set the goal, "Students will be able to answer questions using past progressive tense," check for success by asking, "What were you doing yesterday?" If the student answers, "I was riding my bike," it's likely that she understands the concept. If your goal is, "Students will be able to ask simple questions using 'what,'" you can check for success even with very weak students by supplying the words of the sentence and seeing whether they can put them in the correct order.