There are entire books devoted to this topic... :)
I'd suggest focusing on a % of income that you can set aside for each child and start doing that instead of looking at a specific dollar target. I would look at a 529 plan to get the state tax benefit for growth.
Also, be careful to counter the "advice" given by admissions folks, who encourage high school kids to "choose the college that will make you happy" and expose them to as many pictures of lovely, leafy private campuses as possible. The lawns at the private school are nice, but state colleges offer a great education at a relative bargain.
Try to teach your kids about money so they understand the downsides of the loans that schools throw at you. I went to a state school, had very supportive parents and worked throughout. I came out with $750 of debt (on a 0% credit card for a laptop replacement). I have a friend who went to a similar, private school who came from a similar background and who is now in a similar place career-wise. Except he has a $500/mo monthly tribute to Sallie Mae.
My parents started working on me when I was about 12, and it worked!