On top of using base flowering nutes I like to use PK13/14 in the 5th week of flowering and "DM Potash Plus (potash,phosphoric acid,K)" until the final flush. I hit them with a flowering hormone ("Buddah bud" 2-Chloroethyltrimethylammonium Chloride, N,P,K,Ca,Fe,B&Mn) this time in week 5 which has caused my colas to double in size while remaining nice and dense. I also like to train my colas to grow horizontally to the light so they get light all the way along the branch to form more budsites and tighter buds. Decreasing the distance between the plant and the light in later weeks also works for me. Sure they get a bit singed sometimes but they love the extra light and fatten right up.
Pic 1 is the end of week 4 of flowering and pic 2 is the end of week 6.