This is something I came up with when I changed the way I held my yarn when I learned to knit. Before I learned to knit I held my yarn between my 1st 2 fingers (kind of like a cigarette). When I learned to knit I learned continental and changed my yarn hold so that it wraps around my pinkie and up and over the back of my hand and index finger and this is a much better hold for me.
For the longest time I had problems holding my yarn in the same spot on my finger. To solve this I created yarn rings. I would get scraps of yarn and tie them into loops around the yarn I was working with. I would make the loop large enough to stay between my 1st and 2nd knuckle.
I then put the ring on my finger and my yarn still runs across the back of my hand and over my index finger but the yarn ring keeps it from slipping off. This is also hand when working with more than one color as you can make a separate yarn ring for each strand it it will keep them from crossing.
Here are a couple photos of the yarn ring in use. They don't need to be tight, it's just a guide. I don't need it like I used to because i have become accustomed to holding my yarn secure but it's a handy tip to pass on for anyone who is having problems keeping their yarn in the same spot on their finger.