I think you answered your own question. The longer you put it off the less motivated you will be later in the evening to finish it. I'm about to graduate college and here is how I have approached it:
1. School doesn't end once class ends, I go home but I still try to remain in the school mindset. (So plan on your workday being an extra hour or 2)
2. Keep music playing in the background as you work. If you do it in silence, it can lead to mind wandering.
3. If you have to be moving around while you focus, chew a piece of gum and try to keep your other hand busy on something while you work (twirling a pen or something)
4. Don't be in discomfort, but don't sit somewhere that is too relaxing and will make you tired.
5. Take it in little steps. 30 minutes of hard focus, 15 minute breaks. Before you know it, you'll be done and you can ENJOY your evening without the work hanging over your head.
Also, don't eat any huge meals before you do homework. Reserve the blood in your body for brain power, not for digestion ;)