
The secret behind getting a perfect white background is to light your background brighter than your subject. I found a cheap dinner table cover paper sheet in Sam's club and it worked as a great background for my picture. To make sure my background is well lit, i covered the glass backdoor in my house with this paper, with the evening sunlight coming right through it.
I set my camera to over expose the subject by +4 ev. That way the subject is still exposed properly even with the bright background. Alternatively, you can try using spot metering on the subject to get the same result. I also used a little bit of fill flash - 1/32 power in manual mode.
Finally, when I tried the shot, my daughter grabbed the paper background and tried to, well, eat it. But that, I guess, made the photo more interesting.