The last imposing hurricane we had, (can't remember the name) I wrote my name, barn address, phone numbers, etc. on a peice of paper (think I scotch taped that, too so water wouldn't mess it up). Put that paper in a ziploc baggie. Duct taped the heck outta that ziploc baggie to a bell boot and put one on each horse. They held up through the rain, mud, etc. for a few days. I didn't have to leave it on that long (because hurricane didn't hit) but I wanted to test it for future reference. Took each one off and opened up the baggie and everything was there. I figured that was better than taping the baggie to a leather/breakaway halter because, of course, if the horse snagged it it would 'break away'...this way, the bellboot is totally safe and lots of horses wear them daily in their paddocks anyways. Of course its never good to leave a bell boot on for days BUT, in the case of an emergency...who cares? :o) 
  Here's a couple more tips: 
NEVER EVER Put your coggins test into the ziploc baggie or on your horse at all. That is your proof that that is your horse. That is the only way you will get your horse back if there is a question of ownership. 
ALWAYS Have current photos taken with you and your horse(s). hee hee..not hard for us on this list, right? :o) We always have pics to share. But in a question of ownership the photos help. 
Other ways I've heard of are: 
Braiding ID tags into manes or tails. 
There are hospital type bands some horse folk sell that go around the leg above the hoof you can put info on. 
Those bands you see broodmares wear around their necks are good too. 
Putting info in a ziploc and taping to breakaway halter. 
Spray painting phone numbers on horses. The spray paint they sell for the endurance riders is best, don't know what is in regular spray paint but it may not be good for the horse's skin. 
Anyone have any other ideas? 
Heather