If I'm doing a treasury for fun, I think of a theme and then find items to match it. I usually start with my fave items and shops, then move to team shops, and then look for new items and shops. I like to feature shops that don't have that much exposure and haven't been in many treasuries. Then I arrange the items in a way that echoes the color pattern and shape pattern of each item until it looks like the items are meant to be together rather than random collections.
Then there are promotional treasuries, which is a bit more challenging. You have to find a link between the items that you're trying to promote.
Also, if you haven't joined it, the Add Item to Treasury from Item page prototype, you should. It's a great way to create quick treasuries. Here's the link:
https://www.etsy.com/teams/13135/add-item-to-treasury-from-item-page