That's why it's called a hack, there is some luck involved along with the perseverance.
In my case I've recently been rewearing a much older less stylish belt as an example of what life was like when Ronald Reagan went from the first low-class actor in memory to become elected president of the USA.
It was just barely tight enough to begin with and I ended up having to poke some additional holes so it would still hold up my pants. For that reason it's been relatively ugly for quite some time, but has never failed to fit.
A common saying back then was "belt tightening of the '80's".
That was 40 years ago, but no better time than the present to break it out again.
I have fluctuated 36-40 (mens US) for the last 20 years. If you are gaining or losing tremendous amounts of weight in a short period of time you should probably talk to your doctor because there may genuinely be something medical going on.
Then keep your waist size the same over the long term as a life hack.