Something I've learned from my woodworking (making a faux antique means you don't do shiny)....

Careful with anything that will leave big scratches. Steel wool and sandpaper have that habit, if you have a very delicate touch it will work.

How I do it though:

Grab some ultra-fine grit sandblasting powder and toss the buckle in with the powder in a bag. Then, toss it on something that will vibrate the whole deal. I use the mechanism from a back massager cushion. Toss in some freshly recharged batteries (I know, saying to use rechargable batteries instead of normal ones doesn't seem right) into the machine, and let it go until the batteries die, or until the buckle is the shine you want.

Then, if you really want the "antique" patina, use some black liquid shoe polish or black ink. Wipe it on, wipe it off, let it dry.