Skinny belts that are too long can still work if you use this shortening technique. It’s nifty and one I’m sure you’ve seen before.
Wrap the long, dangling end around the part of the belt that’s on your waist and pull it through to create “a knotted effect”. Pull the knot tightly and the belt stays put all day.
If the ends of the belt are too short to knot, but long enough to dangle, use double sided tape or Velcro adhesives to keep the belt end in place. Having belts professionally shortened by a cobbler is another route to go. But if you’re short on time like I was last week, knotting works just fine.