Encourage young people to try new things
Sometimes people discover their passions when they learn something new or do a project in something they'd never studied before. I think the less instruction people have, the more it gives them an opportunity to learn and discover rather than regurgitating what's expected.
Give them a good grounding in reality
Too often, people have to choose a field of study or an occupation with little concept of what the future holds. They might not have any clue about what a mortgage costs or how many women end up single or what taking a career break can mean for their future incomes. Help them understand what they need to prepare for.
Don't impose your own limits
This is important because if we teach people to follow gender stereotypes or to do what their parents did, they might end up in a career that's not suitable for them. As the world changes, many careers will exist today that didn't exist in the past, and vice versa, so our preconceived ideas might not work in tomorrow's world.
Teach them not to close doors until necessary
Eventually, they'll get to the point where they have to sacrifice some things for the sake of time. But if they reject some options too early, it will be more difficult later to play catch-up (if they stop taking science classes in high school and then decide to become physicists after college graduation, it may not be impossible to do, but it will be much more difficult). Speaking of which...
I believe that the more education a person has, the more options he or she has. This is because when a person is better qualified, their skills become valuable to employers. Then the person has more job options, not only in terms of what they do and who they work for, but also in terms of when and where they work.
It's never too late
I acknowledge that it's much harder at certain points of your life to change direction than at others. If you have young children, for example, it's not going to be easy to go to school full-time. But that doesn't mean school is impossible. Rather than convincing young people that the choices they make in their early twenties are carved in stone, remind them that they can reevaluate their options at almost any stage. If they hate their school, they can transfer. If they want to go back to school, they can do it online or part-time if need be. If they want to change careers, they can take steps toward doing so while working somewhere else. Life doesn't have to be neatly divided into phases such as "school" or "working" or "parenting"--I think it makes things more fun to mix them up a little.
You can't change the choice you've made in the past, but if you evaluate the options you have now, you can make changes, even if those changes are slow and incremental.