One word springs to mind: bribery!  It takes maturity for a student to realise the intrinsic benfits of learning and until that time most kids will work hard at their studies for two key reasons: to please their parents and/or teacher and to get rewards that are somewhat more immediate than good career opportunities in ten years time! 
Key Motivator Number 1:  Pleasing you and their teacher.  Lets face it, some children are more driven to please their parents than others - but most kids get a real buzz out of making you proud.  It is important to make pleasing you a rewarding experience.  You need to show excitement when they make progress, to praise them and to show them you are proud of their learning.  Let them know that their school work is valued. It's the little things like telling relatives how well they are doing in front of them, giving them high fives when they complete tasks, attending the parent teacher nights, putting the spelling certificate on the fridge etc.
Key Motivator Number 2:  As children age, often impressing their peers tends to take priority over impressing you.  This is something you can exploit!  If they want to go to the movies with their mates, have the clothes their peers have, spend time interacting online, have their friends over for a sleepover...they need to get their homework done first.  There is nothing wrong with using rewards and punishments to motivate your child. 
Key Motivator Number 3:  Real-life consequences.  Take time to remind your kids of the long term benefits of study - it will sink in eventually.