In reply to Climbing Pieman:
Thread as normal, but have it 'inside out', then thread each end past one belt loop, before pulling the ends together then threading them as normal, so you have a little smaller loop which pulls the first loops either side of the trouser button towards each other.
The extra friction might help stop it working it's way undone again.
It's a method I worked out after I lost my trouser button and my trousers kept 'opening up' around the belt I was wearing, but it might work for you too. (
)
Post edited at 19:01