Enter one of your revolting provinces (not currently under siege) and check the province information window. (Press the church/totem pole/whatever)
Let your mouse pointer hover above the revolt risk percentage value and it will show a breakdown into factors.
If you have taken that province within the last thirty years (and it's not a pagan religion province) you will suffer nationalism of 1-3 % revolt risk, that is also a minimum revolt risk. This cannot be affected, but it will end.
Having wrong culture (not having provinces culture as a state culture) can't be changed either (some exceptions, but only through historical events). This is +1 revolt risk, +2 if you are at war.
Having built tax-gatherers gives +3 and is not something you want to do in a province if you are going
really expansionistic. It should anyhow be preferred only in provinces with religions you tolerate since it is such a large boost. This can't be changed either, but it can be reduced by 1 (to +2) by building a law council in the province when you reach infrastructure level 5.
Stability is an easy issue, stab -3 equals +1 revolt risk, then every increase in stab reduces revolt risk by one until +3 stab equals -5 revolt risk. So keeping a high stability is
good .
And finally, the real crimp in expansionistic policies. Religion.
Press your shield in the top left corner of the screen and you'll see your kings name and statistics in the small screen to the left. Press the shield that contains the religious symbol (some sort of cross if you are christian, some crescent if you are moslem etc)
Now you will have reached the 'religious slider page' Here you move the sliders for those (that) religion you have least represented in your kingdom to the far left. In exchange your other religions move to the right, decreasing revolt risk in those provinces. Fiddle around with these sliders until you get the hang of them. Just remember that changes only take effect when you leave the mini-window, and that every time you leave that mini-window you get a revolt check in all provinces
Did I forget anything (important)?