They're like irregular verbs - students just have to memorize whether they're separable or inseparable. No rhyme or reason.
I require my students to memorize that along with the definition. I've found the best way to re-enforce it is by writing sentences. It's immediately apparent whether they understand which ones are separable or not.
What they seem to struggle with more is how pronoun objects have to be separated. They tend to sidestep the question of separability by just always choosing not to separate them. So, you'll probably have to focus on pronoun objects more.