We opened 529 Plans for both of our kids within a couple of months of their births. We selected the Vanguard 529 Nevada Plan and invested in the Vanguard 500 Index Fund. We choose Vanguard because we have Traditional IRA's and Roth IRA's with them and their expenses and fees are among the lowest in the industry.
The minimum initial investment was $3,000 and the minimum for additional contributions is $50. Our son is 3 years old (Sept birthday) and has $11,020.92 in his account, while our daughter is 6 months old (May birthday) and has $3,457.07 in her account. We encourage our families to contribute in lieu of birthday and Christmas presents, but have only been mildly successful at convincing them.
In addition to the 529 Plans, we have a Upromise account that allows us to earn cash back for college on eligible purchases. This is achieved by shopping shop online through upromise.com or registering credit/debit cards to your Upromise account and making purchases at participating restaurant, stores, gas stations, etc. You can register credit/debit cards of family and friends as well.
We haven't taken advantage of Upromise as much as we should, but have accrued $410.41 so far, of which $396.94 has already been automatically transferred into our son's Vanguard 529 Plan that we linked to Upromise. Free college money is good!