What is the best way to do a keeper league?

We can keep 3 players towards a 6MM cap, with the option to keep a rookie for free (given we started them 3 or less times during the regular season). We lose the pick of the round where the player was drafted the previous year, and can keep them as long as we are able to under cap. 5MM for 1st rounder, 4MM for 2nd, etc. (5th-15th 1MM). Cap number increases 1MM each year kept. Works very well.