Plan your curriculum out ahead of time, and write lesson plans well in advance. This will translate to less work in the moment, so you won't have to worry about the added work of making lesson plans as you start to feel sluggish and slow.
Plan a unit that you love to teach, such as the rain forest or a unit on spiders, during the time in which you are most prone to burnouts. When you have something coming up to look forward to, you will be a better teacher because you will have that excitement coming up.
Schedule a vacation to look forward to sometime in the middle of the year. Even taking one or two days off, and scheduling something fun, like a cruise or a trip to Vegas, can help keep the doldrums away and give you something to be excited about.
Rearrange your classroom in the middle of the year. A new, fresh environment is a good way to keep you motivated and to keep every day from feeling the same as the day before. You can also try buying new supplies like folders, notebooks, and crayons to brighten up the educational process.
Set goals that you can meet to encourage yourself each day, and reward yourself for meeting them.
Take some time for yourself. Take a personal day, or even a half day, and do something fun that helps to pamper you. These little pick-me-ups can go a long way to helping you feel better about yourself and your life.