Try to keep your elbows more or less pinned to your sides. It seems claustrophobic, not moving them much, but try it anyway. I found it helped me make better throws during longer runs. More generally, watch people doing tricks you want to learn. This buys time, and keeps those throws out of the way of the bad one.
Most people prefer beanbags to balls.
Many find tennis balls too big and too light and bouncy, to be good, but some sand, water, or pennies in each one tames them, and keeps them from rolling everywhere to boot. Silicones are expensive and dangerously bouncy, but feel sooo good. I find them too heavy and dangerous for 8 or more. Only consider where they hit the ground or better, the sand, and not where they roll to. Throw ONLY, and do your best to clear your mind of any thoughts of catches.