I learned mostly from online videos, and a few things from people at the local spin jam. Mirrors are helpful for checking things (your planes, timing, etc.) but can become a crutch if you over-use them; still handy to have one available, though. Better still is to find a spin jam in your area, if there are any, and hang out with other spinners. Sometimes seeing people doing something in person completely unlocks some barrier, or they might be able to spot some mistake you're making that you can't see or feel. Lacking a spin jam, mirrors and perseverance are your best friends.
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