ESL classes are organized by the level of the learners, but within each class level there are always gaps in comprehension between students. You need to be mindful of this while planning your lessons. You should also think about how well your students seem to understand the textbook and about how useful the textbook is in conveying English to your particular students according to their skill level. It's no secret that ESL textbooks can be insufficient material for, say, a two-hour ESL lesson, so take all of these things into account before starting your lesson plan.
ESL teachers need to review the textbook and examine the lesson they'll be teaching next, but also review the lessons that follow it. ESL textbooks are often deliberately repetitive so that students can hear the same grammar points and vocabulary a few different times. So, don't plan to focus your whole class for a day on a point that will come up every day for a week.
If you have access to a computer in the classroom, try to think of songs or videos that you can find online that will tie into your lessons. You don't have to resort to standard ESL videos, especially not with high-level students. Show movies that deal with the same subject matter as your lessons -- or clips from those movies -- and then work discussion time into your lesson plans. Don't be afraid of games, either, even with adult learners. Games break up the monotony of a grammar-heavy lesson and relate the information to your students in a new, fun way.
Finding a template you like and can stick to when making your lesson plans will save you a lot of aggravation. You can find tons of ESL lesson plans online, or you can build a class plan template that works for you. PBS breaks its ESL class plan into the following sections: overview, materials, objectives, time required (for each section), procedure, evaluation and comments. Try using each of these sections until you find which ones work for your classes, then change the lesson plan to suit your individual needs.
Don't feel the need to stick to the letter of your lesson plan if the class goes in a different direction. As an ESL teacher, you need to adapt to the circumstances but can use the class plan as an anchor to return to. If the class is going off-topic, check where you should be with the lesson plan and rein the discussion in, then move on to the next part of your plan. But if the discussion seems fruitful -- if the students are taking a lot of vocabulary away from it or if they are speaking more than usual -- just roll with it and come back to the lesson plan later.