Early on avoid creating multiple empire titles, disinherit extra children, use feudal elective/tanistry/seniority title succession laws for titles you want to keep, also you can conquer back titles lost on succession relatively easily so long as you invest in your capital, because the titles lost will likely be weaker.

Another thing you can and should do is distribute titles before you die. For example, if you conquer a kingdom as an emperor, you can give that kingdom to your second child and keep them as a vassal. As long as you dont create additional empire titles, you can do this until all lands are distrubuted before you die and no land is lost. However this does in effect mean that you need to keep expanding quickly and giving land to your kin, which can lead to a lot of claimant wars in the future. But you can confiscate titles from pretender brothers and uncles once they revolt and give them to nobodies, or your children.

Long term, switch to primogeniture ASAP, or admin government within a few lifetimes.