So I have quite a few fusion cores but these two tips will apply to anyone with a fusion core.

Basically when you see your fusion core about to run out you should remove and sell it. With the right charisma mine are selling for 160 a pop. So rather than it just disappearing save it for caps.

The second option is to drop is as a nuke in your power armor. But do not use the cores for this until 10/100 or less. Take it out and replace it. When you get to a tough spot just switch to that core, pop jet, run in and drop it in the middle of your enemies and RUN out. I've killed 4 at once with this.

Hope this helps!
Thanks for any feedback