We use Sistema bottles for our two dc and at home it's easy to tell which bottle is which because they have their own colour bottle, but for school and clubs they really need to be identifiable. I put stickers on them and then loads of sellotape over the top which works but quickly looks disgusting after a few washes, so I bought a sharpie pen that supposedly writes on anything, but the second the bottle gets wet the sharpie just wipes off!
Any clever tips?