I usually tie a HUGE piece of yarn doubled-over several times because I often fish medium-large flies and a tiny piece of yarn is too easy to cast for practice purposes.
were eyewear when casting a fly rod. Always
Sorry to show this because it has scarred me for life, but you need to know why and this way is the most effective to get the point across --
Why you should Always wear Glasses and Only use Barbless hooks. « the limp cobra
I was always pretty good about wearing sunglasses while fishing -- but I will never, EVER fish again without them. Not even casting practice. If I'm holding a rod, I have glasses on.