One solution is that you don't retain them. You resign yourself to the fact that you'll constantly be hiring junior people, training them, putting them to work while underpaying them, then losing them. This is the principle behind "churn and burn consultancies" and while they are no fun to work for, they can be a good stepping stone to a better job.

For the record, my first programming job was a churn and burn consultancy. While I hated it at the time, it turned out well in the long run. I was there 10 months and my salary went from $33,000/year to $45,000/year. My next job paid me $70,000/year.