For starters, it doesn't happen always because I've exhausted all generic search terms. I've been collecting hair related stuff for some 20 years! So it's very easy to hit a saturation point.
But the key is consistency. I browse for stuff when I'm bored or horny. Nowadays, unless I come across a new platform, the real key is "indirect search." By that I mean, if you just search for "ponytail", you may not find something new. So I'll start off with a generic search or even a profile and when I find something interesting, I'll check out if the content is in any other albums or groups. There are a ton of people who aggregate stuff, either as their favorites or explicit albums, post to groups etc.
And guess what, once you start branching off, it can quickly get out of hand. One picture will lead to an album and suddenly you'd like to check out that entire album. Then you'll be tempted to check the user who made the album and it's not unusual for them to have literally thousands of pictures. At this point I'll just bookmark for another day.
It also helps to have a good computer setup. I use mouse gestures, so many common actions like opening a link in a new tab, closing it, switching to left or right tab, copying urls etc are mapped to easy mouse gestures or buttons (I've a Logitech MX Master mouse.) Since downloading from Flickr can be a pain, I've an extension that copies all URLs and then I paste it into a small program I wrote that will filter just the Flickr links and download them (using gallery-dl program). I've a similar setup for YouTube.
All these might sound like a lot of work but I've had so many years to hit this sweet spot. Not that it's difficult in any case!
I'm sure that as long as Flickr exists, I'll never exhaust that treasure trove. So many hidden gems. Maybe in the future AI will help us automate this manual labor but I suppose hunting for new stuff, so to speak, is part of the fun.