I ask each owner to supply a waterproof (and preferably breathable) turnout sheet. I prefer the ones without insulation - just a plain waterproof sheet. 1200 denier or better is really necessary, as anything less is likely to rip (or be ripped!) in our group turnout situation. I've had good luck myself with a number of brands - my horses are currently in Weathabeeta Oricans that are several years old, but Jill got a more recent set for Scout and Joe and the fabric just isn't holding up as well. It's also good if the sheet fits and provides adequate coverage on the sides, legs and tail area - a horse with a sheet that doesn't fit can be pretty uncomfortable and can even end up with soreness - say at the withers or in the shoulder area - that can carry over to riding. I have my sheets and blankets washed, mended and rewaterproofed every year, and they usually last for several years. I noticed this morning that several of the horses have sheets that are clearly no longer very waterproof - I'll have to point that out to their owners.
I sheet the horses in their stalls as they are eating their hay before breakfast - with one exception - Dawn gets sheeted on crossties before turnout as she can be worried in the stall. Some of the horses used to have problems with sheeting, but we've worked through that. Scout would race in circles around his stall - approach/release worked really well to convince him that he could stand still. Charisma used to be afraid of her sheet, and would also bite when it was being fastened. One day I just dropped the sheet on the floor of her stall - she wasn't afraid of it when it wasn't moving or otherwise I wouldn't have done that - and let her finish her hay while she kept an eye on it. After that she wasn't worried about it moving. The snappishness was cured by giving her lavish praise when she kept her mouth to herself - in her case corrections for aggressive behavior tend to escalate, but praise works instantly. She and Scout are now the perfect lady and gentleman.
Both Sugar and Misty can be somewhat snappish - I believe in part because they just don't like the way their sheets fit. In their case, I just stick out my index finger and arrange for the soft side of their nose to "accidentally" run into my finger as they try to snap. Misty then bites the wall, but I don't care about that if it helps her.
An interesting thing with Dawn - she's always been a bit crabby about having her sheet put on and taken off - she tries to bite the sheet and also doesn't like the front straps to be fastened - and her sheets fits really well. This morning, she was ears up and relaxed the whole time, which was a first. A different horse, really - I don't know if it's the Mare Magic, the magnesium, the ulcer meds or just the fact that she's just more relaxed in general. Whatever it is, I'll take it!