I'm not a stable owner, but I think that in general a good boarder:
- follows the rules/regulations set out by the barn manager
- is open and honest with BO
- makes sure there is a mutual agreement (preferably written) between the two and knows what is expected of her as a boarder
- tidies up after herself when grooming, etc.
- asks before using the BO's tack or other equipment
- takes responsiblity if she breaks something, etc.
- works cooperatively with other boarders
- has good horse sense and management (this is important because the boarder may have to deal slightly with other boarder's horses)
- doesn't bring friends onto the property without asking first
- pays monthly board on time and in good order (this is a biggie, which I should have mentioned first)
That's all I can think of for now. This is basically what I've gathered from being at my barn, seeing both good and bad boarders. I've always tried to be a really good boarder because the BO is extending their hospitality out to you, and its just general courtesy.
Hope that helped a bit!