Hot air hair dryer, and fingernail or old driver's license, carefully.
As the decal or sticker gets hot, the glue gets soft, and it's removable.

Extreme Goo Gone works well afterwards.

I've also soaked a rag in Mineral Spirits and gotten the decal good and wet with it, then wiped it off.
A few seconds later, the decal will often start to deteriorate. Be careful, or it could take paint, too.
You can repeat that process a few times.

Basically, diligence and perserverance, and you can have a nice frame with no decals.