Top tip is to buy a multi pack of stainless ones with a mix of different sizes.
The plastic ones aren't great and you often seem to loose one of the pair. The they're usually only preferable if you have a thin/fine summer shirt - say one with a high linen content. Then you risk damaging it.
How have I never known about these?! My favourite shirt has an annoying floppy collar - are you saying that there's a solution to this??