When I was in third grade, a kid in my class told me that Michael Jordan was so rich that he used a toothbrush only once and then threw it away. I asked him why he would do that and he said that new toothbrushes are way more effective than ones that have even been used once. I tried looking up "Michael Jordan" and "toothbrush" on Google, but all I came up with was Jordan's new toothbrush mustache, also known as the Hitler mustache.
But I found this paper, along with some others, that did a study to compare the effectiveness of a brand new toothbrush and a 3-month-old toothbrush. Manufacturers recommend that you replace your toothbrush every 3 months, and these people wanted to see if it really made a difference how old the toothbrush was.
Result: there's not much of a difference.
I guess the big issue is how well your technique is when you brush your teeth. The thing that grossed me out the most is that they had the people not brush their teeth for 48 hours and then they measured the amount of plaque that was removed by brushing their teeth. The gross part is that they only removed on average 34% of the plaque on their teeth, regardless of the condition of the toothbrush. I guess that's why it's important to floss, too.