I use a small paper binder clip that I keep on my keychain so it's always with me.

paperclip..JPG

When I use it I fold my pants over my ankle and clip the folded end to my pants and push the clips back up into my pants.

paperclip_pants..jpg

It's nice because it's really small and always with me so I dont' have to worry about losing it - not to mention it's really cheap.