I use alfa-MEM with 15 to 20% of FBS. Seed cells at around 10-15% of density, split once a week, never let them go more than 60% of confluency (at high confluence, they start to transform, lose stemness and spontaneously differentiate). Also, for primary cells, there is a limited number of passages in culture, you should not keep them for more than 4 passages (in general). My protocol works very well for me.