The concentration span of an adult is only 10-20 minutes long. Apart from changing your delivery style frequently, you also need to learn how to provide state changes for the students if you want them to stay engaged in their learning.
'Energisers' can be used as a state change if you have time and make it relevant to what you are teaching. The main goal of an energizer or state change is to get the blood flowing and wake up your students, or to bring some humour to your class. Three important ingredients to learning well.
If there is not a lot of time and you just need to re-focus the learners try any of the following:
Using a an optical illusion might be a quick state change. Check out www.planetperplex.com for an endless supply of free resources to use. Find relevent images to what you are teaching and keep them in a folder, ready to use when you need them.
Make student start an exercise in their workbook. Half way through, say firmly "Stand Up" and make the movement with your hands. While they are standing you can either do a quick left brain/right brain activity or you can simple ask them to sit down again.
Brain teaser as a quick state change. What goes up and down most likely several times a day but it never physically moves? Answer: temperature
Walk to the other side of the room and start teaching from a different angle. This means students might wonder what you are doing, turn their necks or body's around that will get the blood flowing again.
Make a mistake on the white board. Guaranteed that one student will say something , or they will start whispering or giggling. Take this opportunity to play the innocent and ask them to explain what went wrong. Be a great actor and have some fun with it. Anybody who was "asleep" will be alert again.
Simply changing desks or groups around, gets students up on their feet and moving, able to have a quick chat and than refocus on the learning.
Have some wow factors memorised to create 15 seconds of laughter or discussion and carry on teaching.
Have some 'impossible to complete' activities like: The first person able to lick their left elbow gets $ 10.00. (go an try it now and be convinced you will never have to pay the $ 10.00). This is quick, moves the body, provides laugher and back to teaching.
Put a quick brainteaser on the boards like "what comes next O,T,T,F, ?,?"
our so the next letter is F