1. Teach things that will relate to their next step in life.
There is nothing worse for motivation than being forced to learn something that you feel will never be useful in later life. So try to find out what the students want to do/be and teach them things that will be useful for that. Luckily for me I teach at two very specialist high schools so the students generally have a pretty good idea of what they will be doing after high school. If your not so sure about your students life goals have a lesson about what they want to do after High School and use some of the ideas you get.
2. Activities where the students can find out about each other.
Why do you think Facebook is so popular? So you can keep in touch with your friends and family from across the world really easily? Or because you can easily be nosy and find out what everyone is doing? Bring the ideas behind Facebook into your classroom by playing games where the students can find out about each other. Some examples of activities you can do are:
2 truths one lie.
Ask each student to answer questions about a topic on paper and hand it to you. Then ask other students in the class to choose a random paper and read it out. The students have to guess whose paper it is.
Competition is a pretty universal motivator as most people really like to win! The key however key is making the activity something the students want to win at. Also whilst it is good to have a winner, try not to single out a loser.
4. Famous People/ Celebrities.
My students ALWAYS get more excited when I link the lesson to celebrities, and luckily it is very easy to link celebrities to a high range of topics. Also if you teach high school girls and you want to see them reealllly motivated and focused, just link a picture of Robert Pattinson into your lesson.
5. Quiz reviews/ introductions.
When introducing or reviewing a topic a quiz can be much more effective at motivating the students than just showing them vocab. I usually show about 4 clues and try to get the students to guess what they are about. A simple example would be instead of saying "This is the Olympics, it is a sports competition that originated in Greece and is now held every 4 years in a different country." you could put the clues 'It is from Greece, It is held every time in a different country, It is held every 4 years, It is a sports competition' on a PPT and see if any students can guess what it is.
If you stay motivated to come up with the best lesson plans possible, the students will undoubtedly notice and take on some of your enthusiasm. Furthermore you are more likely to come across as passionate when you teach and the extra thought that you have put into your lessons will make the activities much more exciting for your students!