Regardless of whether you're using a mouse or tablet, try to make the diameter of the circles you spin as small as possible. That is, don't move your mouse in huge circular motions but rather small, "concentrated" circles focusing on the center of the spinner. It's a bit like what cookiezi does in this video: http://youtu.be/_HU_S1VkG9k?t=58s
Also, watching this video 24/7 has been scientifically proven to improve osu! spin speeds kuru kuru kurukuru~
Start by spinning slowly, but in a good, consistent circle. Doesn't matter what size the circle is as long as you have a consistent movement. Now after you have a consistent circle, start moving faster and faster while maintaining your consistent circle and keep practicing.