At various times I wear either a fly plaid or an Irish brat. With both, I pin them; a large safety pin just below (anterior to) the brooch, and a small one at my shoulder (both pins inside the coat). There's just too much activity going on on my left shoulder to leave the thing simply flapping about (a brat would just fall off), and one of my brooches has this great spike on it; not something I want to poke through a jacket.
You could replace the spike with a piece of piano wire; my favorite brooch came second-hand (one of my best EBay finds) without it's pin, and that's how I repaired it. Don't sharpen it too much; you want it pointy but blunt enough to separate the fibers without cutting them.
Never leave brooches, pins, kilt pins, etc. in place when not in use. Pewter and steel can oxidize, permanently damaging the cloth.