I've been a runner for more than 10 years. During that time, I've sporadically done yoga and Pilates. Last year, I started consistently practicing yoga once a week. I'm still incredibly inflexible (physically) but my upper body is very toned and I've shaved almost a minute off my time.
Yoga gear for runners:
Some gear is best for just yoga, but if you're a runner, you probably already have pieces that can go both ways.
Top: Sleeveless tank or singlet
I prefer a tank with built-in sports bra. One of my favorites is Power Y Tank by Lululemon. It has removable padding that hides "headlights," and the Y shape of the straps makes them comfortable in every position. The guys in my practice usually wear Under Armour or Nike singlets.
If it is cold in the studio, I might start with warm-up jacket, but good yoga is not just stretching; it's fairly active and I warm up quickly.
I don't like sleeves when I do yoga because of the wide range of arm movements being practiced. I want to feel free to do my best. I also wouldn't go shirtless, if I was a man or wear just a sports bra - the feeling of the mat against bare, sweaty skin is not nice.
In the summer, I wear fitted, stretchy shorts. My current favorites are the Chaturanga Yoga Short by Athleta. They have a 3-inch inseam, are very stretchy and usually don't get wet marks from sweat.
In the winter, I usually wear a boot-cut yoga pant or Nike's Dharma Yoga Capris. It's tempting to bundle up with a thick material, but I don't recommend it - when you sweat, it will get saggy and soggy. Stick with a wicking material that moves with your body and covers your bottom, even if you have your feet behind your head.
Happy running, yogis!