BUDGETING. We did it by budgeting everything and using their savings.
We did exactly the same thing that @happy1 did.
We shared our annual budget with each child to inform them about the costs of colleges and gave them a college budget (when they were each freshmen in high school) and had them search for affordable colleges. They could apply to two expensive colleges. If they got enough merit money, great, if not, no go.
We saved $25 per month, per kid in their 529 plans, since toddler age.
We sacrificed luxuries and have put off on repairs that we couldn’t do ourselves.
Limited vacations -camping to save money. We both worked overtime and some weekends.
We made each child work their summers in high school and college to earn money for their incidentals. They took out their own college loans, that they repaid, each year with summer and on-campus jobs.
Each child kicked it into gear at their schools to get good grades and high SAT/ACT scores to qualify for merit monies.
It is about setting limits to what is affordable.