Whenever I buy a belt, I'm torn about what size to get: large enough to sit on my hips to wear with jeans or other pants, or small enough to wear at the waist over dresses, skirts, or high-waisted pants? And whatever I decide, it's only a matter of time until I end up standing in my closet, half-dressed, wishing I'd gotten the other size.
Then I learned a great trick to make a too-big belt small enough to buckle neatly around the waist: just tie a knot in it.
This couldn't be easier: take your too-big belt and tie a simple knot (just like you do when you're starting to tie your shoe) in the back of the belt to shorten it. That should take out enough of the slack so that you can buckle it normally and wear it up on your waist.
If you're wearing a blazer or sweater that will cover the back of the belt and the knot, you won't need to worry about where the knot is hitting you (and it's actually a bit more comfortable if you have it slightly off-center, so that when you're sitting in a chair it's not poking you in the back). If you're wearing your belt over a top or a dress and the knot is visible, just play around with the placement of the knot so that it's centered on your back. Or you could put the knot in front, near the buckle.