I think a good work ethic comes from a combination of fear, desire, and expectations (not necessarily in that order). Everyone is a little different, but I will try to give some examples below.
As far as expectations are concerned - look at something like school grades. Make good grades be the expectation, don't celebrate it like it is some great accomplishment unless it really is. I know I didn't celebrate high school graduation and I didn't even get out of bed for my undergad degree ceremony - it was something that was expected. My reward was that I could keep living a pretty good life and have a leg up on those that didn't do those things.
When it comes to desire - goals are great motivators. At a certain age in sports, the good kids will play, the not so good kids won't. If you want to play, most will have to work at it. If there is an aspiration for a certain degree, profession, etc. Cultivate that. Make them want it and understand what it takes to reach that goal.
Fear - This really pushed me from a professional standpoint. While I had friends that would get summer jobs at a country club, a pool, etc., I worked on a roofing crew, installing large appliances, as a lube tech at a garage. I was treated like crap, worked hard in poor conditions, and took home very little pay at the end of the week. My father was honest about our lifestyle and what it took to live the way we did (which was very well). I knew from then on how I didn't want to live and had to figure out how to work harder and smarter than everyone else.
