See these articles for more info:
Why Multitasking Isn't Efficient - WebMD
The Myth of Multitasking - Brainworld Magazine
Encyclopedia of Educational Technology - Multitasking
Human Performance - Predictors of Multitasking Performance
Try focusing on one thing for about 15 - 30 minutes, then giving yourself a short break, a reward, or switching to another task. Even moving to another location 2 - 3 times an hour will help you get more done and stay focused.
Adding some music (especially music with little or no lyrics) can also be helpful to set a mood for studying and doing homework.
Here are a few songs and sites I enjoy listening to while I study or work on homework:
ATB - Beautiful Worlds
Nujabes - Aruaurian Dance (Homework Mix)
Aersia - A miscellaneous video game music playlist
Aersia Mellow - Light rain + soothing video game soundtracks
Emancipator - Ocelot
Emancipator - With Rainy Eyes (Extended 1hr Edit)
Ibiza - Trance progressive, non-vocal Good for focusing on work that requires some thinking, language
Melodic dubstep: https://www.youtube.com/watch?v=p6j8fuvQICI Good for working on semi-cognitive but fairly boring stuff
Incredible Vocal Trance June 2013 Mix 2 Pretty good for spreadsheet editing or other repetitive tasks, fairly non-disruptive
...or if those aren't your style, try searching YouTube for things like "homework playlist", "concentration playlist", "study playlist", etc.
Of course you could always build your own playlist of songs that help you focus better, and even customize it for the type of work you need to do (e.g., songs to get you pumped, songs to help you stay on task, songs to promote creativity, etc.)