Every time, after I finished a math problem, I'd take a bite of chocolate. I'd tell myself I wasn't allowed to eat more chocolate until I finished the math problem.
I found that it worked really well, and I was able to get through my entire math packet without having a hard time being able to motivate myself to do it. At least for me, math is not my favorite thing to do, but if there's chocolate involved it's much more enjoyable.