How do I not run out of steam in high school?

At the start of the year I begin with a lot of excitement and readiness to get good grades. I'm organized, complete all my homework and go above and beyond the requirements. By about February...I start losing steam and not turning things in, coming in late, getting disorganized...and by the end of the year, when work REALLY piles up, I lose all my enthusiasm and just barely pass. I hate being in this pattern and I want to break it. But how do I do that?