Worshipers and worshiping, they need lots and lots of both.
GW leaves the exact mechanics vague (probably) in order to give their army of authors some leeway when writing new army books or novels / short-stories.
It differs from 40k to Fantasy too, but generally the Chaos gods need a huge amount of worshipers doing whatever it is that particular Chaos god really likes (violence, sex, being really sick, changing stuff?) for quite some time before enough energy gathers in order for open a gateway to the warp, then out come the demons.
There are literally an endless supply of demons in the warp, but the problem is, each one can only last a short time on our plane before they expire, I suppose each one has a built in battery which they use up pretty fast.
AFAIK, it's never really explained why a demon can only last X number of minutes or hours outside the warp. Our plane of existence is probably toxic or too clean for them.
Greater demons can last longer but they need constant death, sexy time, disease or change to sustain them while they're here.
It's only in a situation where the wall between our reality and the warp has been completely broken down that you'll see huge prolonged incursions. The End Times for example, or the Eye Of Terror. These situations usually require some massive cataclysm so we don't see them very often.