I do realise this is not the way professionals go about it, but I'm a happy hobbyist playing with what comes up in my mind and liking experiments. Here we go:
If I want to make a scent last longer without changing it, at least not changing it much. What can I put in it?
Glycerine has been mentioned as a fixative but plenty of people seem to dismiss it. Someone mentioned it works for the fragrance molecules on the water soluble side, but not the rest.
Isopropyl myristate is mentioned as a fixative on Wikipedia and on other places, but you don't discuss it much on this forum so I guess not many of you are using it.
What else? Propylene glycol, dipropylene glycol...?
In an old thread someone mentioned that old perfumes sometimes might contain a little castor oil. I suppose the amount needs to be very tiny, but if the general idea is big molecules a guess a small amount of fat could do the trick. It sounds right to me. I guess no water should be added (and none of my home made tinctures that probably have been diluted by water from the plants) or the oil will quickly separate.