The far best way of getting time to do homework (day-time that is), is to clear away any distractions. If your habits are anything like mine used to be, you probably turn on the TV as soon as you come home. If you try not using TV for a few weeks you'll find out that your life is not dependent on the box in the living room (or your bedroom or whatever). The same goes for random internet surfing, the chatroom, facebook etc.
2 - Get more self discipline/get more organized
When you get home from school, do most of your homework at once. Don't let anything else distract you (within reason of course, if your house is on fire or your grandfather is dying from cardiac arrest, you might want to postpone your homework a bit). If you get bored turn on some light music in the background (NO TV/ Radio!). It is important that this music isn't too intrusive. Get some food while you're working (you're probably hungry when you come home from school anyway).
3 - Work out more
Physical training gives you more energy on a long-term basis. Thus, giving you more energy to do your homework. Combine with step.
4 - Eat healthier
For the same reasons as step 3. Hamburgers and candy bars are NOT the ideal food for a creative and working mind.
5 - Go to bed earlier
Nobody feels fresh and ready for a load of homework if they have slept for 6 hours that night. Try to get at least 8 hours of sleep every night, not fewer than 6-7 hours.
6 - Tell your parents about your bad habits.
Make them nag you until you do your homework. This might seem a bit self-destructive, but at least you'd get some support.
7 - Get anti-social
Try not to exercise a social life until you've finished your homework. You'll get plenty of time to socialize, afterwards.
8 - Don't postpone things.
Don't postpone until tomorrow what you might do today etc etc... . You'll probably end up doing it at an inconvenient time anyway.
9 - Divide and conquer
Don't try to swallow it all at once, divide your homework into bearable segments.
10 - Get good grades
The best motivation there is. Ex: My history teacher gave the top grade on my first test. After that I've had to work like ... . to keep it up. Therefore, I have to do my homework.
11 - Choose interesting subjects
If you're studying the right subjects (ie: subjects that interests you), you'll probably be more motivated.
12 - Don't take too many courses or extracurricular activities.
Ex: Right now I ought to be studying for an English test, but instead I'm hopefully helping you. If you've got bad study habits on top of this, it's just not the right thing to do.
Hope you will study better. :)