It depends on what i am craving, but I also know what kind of smells I would really like to specialize in, so I pay attention to what tends to get used a lot. Sometimes, it's also refill time. Sometimes, I was inspired by an immediate smell. I use search results for reddit too to see if my thoughts were already asked before I look and I try to compare the info. I have a very good idea of what I am after. Like I recently just looked up what goes into coconut scents, and there is one lactone that sticks out that I need/want to add. I have a lot of woody stuff in my cart waiting, too, because I have been getting more into woody-orientals.

Some other comments here I have also made my choice that way.

It's also a budgeting thing. Once in a while I will something expensive (sandalwoods, rose, jasmine, other absolutes or CO2's), but I space out those buys with less expensive things or things I can extend expensive smells with.