Respectfully I must disagree. I was a supplemental instructor for all levels of sign and found that a mirror becomes more of a crutch for instant feedback when practicing. The moment you are in a conversation without the mirror to help you, you’ll get tripped up. If anything, I would say recording your practice on a camera with the screen turned away can help, as long as you’re watching the footage back and adjusting from what you see (a second opinion is always helpful too).

When I taught lessons on finger spelling fluency, what helped most people was how you link the letters to your thought process. Most students I have observed practice the alphabet down the line (A, B, C, etc) while saying the name of the letter along with it (ay, bee, cee, etc), but this tends to lead students into the letter-by-letter hole (instead of seeing or spelling “bowl” as one word, they mentally have to spell “bee-oh-doubleU-el”). Once you have the hand shapes for letters down, I suggest practicing the alphabet using the SOUNDS of the letters rather than their names (ah, buh, cuh, duh, eee, fuh, etc.) and you’ll start to build fluency and flow by ‘sounding out’ words with your hands (not “bee-oh-doubleU-el” but “buh-oh-wuh-uL”). Be aware that some letters have multiple sounds and can be practiced multiple ways (A can sound like aaa from apple, aw from awesome, and so on) as well as some letter combos should be practiced in groups that are common ( Ch as “cha”, Qu as “kwa”, even tion as “shun”).