If you give them a reward system for doing their homework, like a 2 hour pass to play on video games, they will almost certainly fall into a habit of doing homework automatically. Due to the fact hat once they see that homework is now worth doing, they'll get it done in better time and quality. Plus if you do think they are rushing their homework just to get the reward, make sure that you mention to them that they're not allowed to play unless it reaches a certain standard.
(If you feel it necessary, you could deduct time from their video game usage, if they hand in homework more than once which is below standard.) Good luck!