First let's talk about your actually part in this game: you have to create the right environment for your children if you want them to be self-Motivated. To do this, you are going to need to watch the message you send them, be an example and be very consistent in all that.
What I mean by the message is what is behind your words and actions towards and with your kids. It's how they will perceive or interpret it. And this is the thing: there should never be any room for interpretation in what you mean when you talk with your children. The message always have to be very clear; no ambiguity. If you tell your kids they can't go out until homework is done but you still let them, you are sending a very clear message: homework doesn't matter. If you make a promise you don't keep, the message is just as clear: you are not reliable and can be manipulated.
Then you need to be an example. This is pretty self explanatory. You can't ask your child to tidy up his/her room when there is three days worth of dirty pots in the kitchen sink. You can't advise your 17-year-old to give up smoking while lighting up. You can't expect them to be on time to class when you are late everywhere you go. I think we all get this one.
Finally, you need to be consistent in being a role model. Remember that all of the above is what your kids watch all the time, from day one. Children observe their parents all the time and they imitate what they see. So if your actions are confusing or out of line with the values you are trying to raise them by, you are going to raise confused kids that are likely to run havoc.
So that was the part that YOU have to play, as a parent. Now let's look at the part that your kids have to play: Face their responsibility in their actions. I've talked about this before but it is a topic that is absolutely essential to successful individuals of any age.
You need to always encourage your kids to question themselves and accept their responsibility in their actions. This can be used in good or bad situations. If your son comes home with a good report card, get him to realize that his hard work, dedication, interest, etc paid off. If your daughter brings back a bad grade, let her become aware that she should have studied more or more regularly. You don't want your kids to grow up falling into the victim mode where it's always someone else's fault, or bad luck or alien entities interfering with their lives. They have to think in terms of their responsibility.
You will undoubtedly raise more mature children who will soon naturally understand the value of the privilege education is and who will make the most of it out of their own will.
Don't waste time, the sooner the better and it is never too late!