I've had monolids up until around grade 9, when I first discovered double eyelid glue. The trend was just picking up. Everyone was like "ewwww what! you're gluing eyelids and poking your eyes!" :\
Despite that, I REALLY wanted to try. But because I was so young ... I had no money ;-; and the eyetalk glue wasn't really readily available in my area/it was expensive (at that time to me :p)
Lucky for me, Daiso opened and they had their cheap eyelid glue available. I picked it up and ever since used it every day for the next 5 YEARS.
The glue is absolutely terrible though. Those that have tried it will probably know. After it dries, it becomes flaky and crunches off if you happen to rub your eye or anything like that. Even if you don't touch it, it will still become flaky pieces attached to your lid :\
So I am in no way recommending this :p just sharing my experience here.
I used this mostly when I'm home after school, at night when I sleep. Basically whenever I didn't have to go out, I would have this on. It was a way of creating the crease and having my skin get used to folding that way.
I started having a semi-permanent double eyelid around the 3rd or 4th year. By semi-permanent I mean that it goes away sometimes and I would have to put the glue on to re-form my crease in the mornings. The daiso glue is great for this purpose. I would put it on as soon as I wake up. Then by the time I've gotten changed, I will wash it off and my crease will be back :D
During this entire time I hadn't really experienced with eyelid tape just yet.
Most of the time now, I have a slightly folded in double eyelid
(I don't really know what this is called in english :\ 內雙?)
The crease isn't super prominent like say Tsubasa Masuwaka's but it still makes a big difference. In some angles it still looks kind of like a monolid.
When I want eyelids like that, I'd resort to eyelid tape. Way back then, the eyelid tape that was available were all super obvious when you used them. Now, there's so many different types - eyelid fibres, all sorts of different shapes/materials.
I usually use the small moon shape ones to create a more prominent crease.
The ones here are super invisible. It was pretty hard to try and photograph them on my lid actually -_-
I think these eBay ones are the same/close to what I'm using
You can kindddd of see it on my right eye here. I only used it on one eye to show the difference between with/without.
You can see the difference in the next photo.
So you CAN get permanent double eyelids without surgery. You just need a lot of patience :D
If you're not a fan of using eyelid glue, eyelid tape works as well. There are even special eyelid tapes to use overnight now...I didn't have that many choices back in the days ;________;
Don't give up!