Generally you want to pick cultivators with high qi sense to promote to inner disciples (40+ is ok, 50+ is good), as that makes them get more qi literally every breakthrough.
As to how you increase it further... Learn basically any manual/skill that says increase qi on it (make sure you transcribe all the major laws into your bookcases when you get them, also keep a eye out for any chance to get more manuals).
Eat the qi increasing medicines/herbs/flux, (you can eat the same one multiple times with diminishing returns based on its medicinal resistance). Some have some bad effects (wicked flux hurts state of mind.. so don't take much of it if you do take some), or might require you to have a large max qi to not die (if it says it will provide more qi then your max qi, then it will cause spirit/qi injuries, if its a great deal higher it will instant-kill you). Earth Flux is good, and not too hard to acquire,, you can buy it from visiting merchants or at auction sometimes, and can adventure for it. Red ginseng and purple ganoderma aren't that hard to find via exploring/trading either, but have less effect.
Also right before your golden core breakthrough, in addition to getting as many +'s as possible in every category, make sure to take medicines to increase qi gain, as many different types as you can without killing yourself.
Once you get the body-changing techniques by having a primordial spirit cultivator you can mold mortals, you can just spend lifespan to take 'decent' stat cultivators and make them pretty good, which makes the base qi sense quite a bit more important then their base stats (also there are quite a few manuals to raise stats)