1. Simply do it. If you fail to plan, you have planned to fail. I have heard instructors say things like , "oh I don't need a lesson plan", or "its all in my head", etc.. these are usually the same instructors who think that the reason they only have 10 students is because they are an "elite" school. Seriously, if you want to be a professional you have to act like a professional - and professionals always have a plan!
2. The smaller and brighter the better. Small enough to fit inside your uniform, but bright enough so it won't get lost! I use bright colored index cards that even a blind monk could see if they were misplaced. Index cards fit perfectly inside my uniform (I've even gotten someone to sew a simple little pocket on the inside of my Gi to hold my lesson plan). There is nothing more annoying than having your lesson plan falling out, or floating around in your uniform while you are teaching, only to wind up a sweaty mess of ink that somehow wound up stuck to your lower back during situps...
3. Synergy. You want everything in your classes to flow and work together around whatever your central theme is. Example: The week theme is Sparring and its a kids class of beginner and intermediate students age 7-12. Ok, you know the beginners have to learn the 4 corner blocking set because its part of their curriculum, and the intermediate students have to learn a jab cross round kick combination. You do a warmup that involves running across the training floor executing 2 blocks for beginners, and a jab cross for the intermediates. During the stretch you can talk about the different uses for the blocks and the jab cross during sparring. Next you could practice a few basic kicks in place, making sure the intermediates perform a round kick. Next everyone could grab a partner and some mitts and for a few minutes beginners will swing the mitts at their partner and block the mitt with one of the blocking techniques and immediately counter attack with a cross and Intermediates will counter with a jab cross round kick combo. Next you could spar and one of the drills they perform will involve a jab cross round but changing levels, punching high but kicking low, and vice versa. To end class we will play a game of battleball and when they get hit they must execute a jab cross round kick on the heavy bag in order to come back in and play again, and the beginners have to come to you and you swing a blocker that they have to use their blocks against. Do you see where this is going? You did one technique many different ways, everyone had fun, and everyone fully understands the combination, multiple uses for it, and got see it in several situations. Now I don't normally just work on one thing for an entire class, but I find if you start your lessons plan with 2 or 3 main points that are consistent throughout the class you will be surprised how easy it is to teach a class that people leave with those techniques in their muscle memory- and they weren't bored during the process!
4. Don't be afraid to throw your lesson plan out the window. Sometimes you plan everything out perfectly beforehand and things just go crazy. Over 50 show up, only 2 students show up, the kids are all jacked up on mountain dew because school let out early, your assistants don't show up to help, etc.. A good instructor is able to recognize when something isn't working and knows when its time to shift gears and try plan B!
5. Plan more activities/drills than you intend to do (Plan B). One of the biggest mistakes I see newer instructors make is when they run out of activities (because they flew through everything too fast for whatever reason) they usually stand there with that look...the look of "what the heck am I going to do now". After years and years of experience teaching I have learned that it never hurts to write down 1 extra drill/activity on your lesson plan in case you fly through everything.
^ Here is a kickboxing mitt drill you can try out in your classes!