I seem to spend my summer knitting or crocheting in parks, gardens and cafes – it is a hard life I know! – and there is nothing worse than chasing a ball of yarn across the gravel or leaf mulch and then having to clean it off before being able to start again. Now I always keep the current ball in a ziplock bag inside the project bag. You can close the zip close either side of where the yarn comes out to keep the ball safely in the bag. No rolling away, no getting mucky. It works well for hand wound balls, factory wound balls, yarn cakes and centre pulls and they are endlessly reusable.
Ziplock bags are great for keeping yarn stored too as it stops dust settling on the balls, keeps them safe from most attacks, not least of which the nasty yarn eating moths *shudder*.
I always stock up when I visit my favourite Swedish meatball vendor, I believe they also do a nice line in furniture too