All you really need to do is give a county to your preferred son and as long as he is the most powerful of your sons he will be your heir. Succession is the easy part, but the decadence is the real problem if you don't give lands to your sons. I personally just give a county to my favorite son and leave the rest unlanded and try to keep the decadence down with Ramadan. Just give your preferred heir the most land and he will inherit. He can kill all the others once your current ruler is dead.
This is what I do normally, the biggest issue for me at the moment is the decadence even with Ramadan, I'm having to war just to keep it down which is a bit silly. My concern is that the last 2 successions where only my chosen heir was landed lead to factions forming around the various other pretenders both times very quickly (before the spies etc can have any effect). The last really nasty succession was barely dealt with as they got a bunch of free troops from a passing cloud and it was touch and go if I could do it or not. I'm trying to avoid that situation, but in character...
I need a cruel ruler! That way I can allow myself to kill them off... Looks like I will be finding a province with plague in it and sending them as generals to sit there until they catch it.
The main thing for me is I'm not so bothered about winning and losing as I just enjoy playing as a diversion, hence trying to play somewhat by what my ruler's traits are; so it does seem there is no way to play a kind muslim ruler and not get hit by decadence issues if I only land my main son... Hmmm... I think I will land all the other sons as counts under the duke-level heir and let him deal with their scheming for now.
Put your sons on the council. If they have a job they don't cause decadence (Unless you have a huge empire). Can be a problem if you have many sons.
For me at the moment, I have the Arabian empire formed, and over 10 sons, plus other dynasty members such as cousins showing in the decadence tab. I can't remember the exact rate of increase as am not in-game right now. I just don't have enough things to give out which leads to a need to constantly expand the borders just to land dynasty members. It looks like I will be forming Spain and landing everyone there under a dynasty king or something similar.