I had $10 in bitcoin that I got for free (back when it was worth a whole lot less) and a few years ago I used half on Ethereum because it looked like it was going to jump up quickly.
needless to say it was not one of my best decisions.
to answer your actual question, I usually have an idea in mind of what I want for it, and will hold until it gets to that point (for example, I'll hold until double) sometimes it works, sometimes it doesnt, but I don't feel as bad because I've held to my decisions.